Here are five different versions of your table:
In the first version of the table, I have just removed the threeparttable
environment. This causes the tablenotes
to spread across the width of all three tabulars. However, I'd not recommend this solution as it keeps the resizebox
and gives you, as you can see from the following screenshot, inconsistent and illegibly small font sizes:

In the second example, I have removed the resizebox
and manually decreased to font size to tiny
. In order to make the tables narrower, I have also decreased the horizontal white space between adjacent columns. As you can see from the following screenshot, the font is a bit larger than in the first example, however, still too small (as least in my opinion):

To be able to use a larger font, I have therefore allowed a linebreak between the second and third table in the third example. The tablenotes are still as wide as the combined width of the first two tables:

Here is a slightly different variant that places the tablenotes next to the third table. In this example, I have also replaced your \hline
commands and empty rows with the rules from the booktabs
package:

Lastly, your table could also fit into a portrait page as shown in the following screenshot. Here I have used a scriptsize
font as well as the booktabs
package:

\documentclass{article}
\usepackage{threeparttable}
\usepackage{pdflscape}
\usepackage{graphicx}
\usepackage{geometry}
\usepackage{booktabs}
%%%%% Example 1 %%%%%
\begin{document}
\begin{landscape}
\begin{table}
\caption{Caption of the Table}
%\begin{threeparttable}[]
\centering \tiny
\resizebox{\linewidth}{!}{
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
%\hfill
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.457 & -0.318 & 0.570* & 0.649* & 1.011 \\
& (0.309) & (0.535) & (0.325) & (0.348) & (0.806) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.045 & 0.049 & 0.048 & 0.047 & 0.053 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.044** & 0.020 & 0.042** & 0.046** & -0.017 \\
& (0.019) & (0.022) & (0.019) & (0.022) & (0.027) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.084 & 0.087 & 0.086 & 0.085 & 0.091 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
%\hfill
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.066 & -0.777 & 0.182 & 0.481 & 1.795 \\
& (0.467) & (0.856) & (0.486) & (0.549) & (1.263) \\
Observations & 204,520 & 204,520 & 204,520 & 204,520 & 204,520 \\
R-squared & 0.028 & 0.036 & 0.033 & 0.032 & 0.045 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.045** & 0.044 & 0.044** & 0.044* & 0.048 \\
& (0.021) & (0.032) & (0.021) & (0.024) & (0.052) \\
Observations & 261,101 & 261,101 & 261,101 & 261,101 & 261,101 \\
R-squared & 0.062 & 0.069 & 0.065 & 0.065 & 0.075 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
}
%\end{threeparttable}
%\begin{tablenotes}
\small \raggedright
There should be some table notes here that go across all three subtables and not cutoff in the middle There should be some table notes here that go across all three subtables and not cutoff in the middle
%\end{tablenotes}
\end{table}
\end{landscape}
%%%%% Example 2 %%%%%
\begin{landscape}
\begin{table}
\caption{Caption of the Table}
%\begin{threeparttable}[]
\centering \tiny \setlength{\tabcolsep}{2.5pt}
%\resizebox{\linewidth}{!}{
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
\hfill
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.457 & -0.318 & 0.570* & 0.649* & 1.011 \\
& (0.309) & (0.535) & (0.325) & (0.348) & (0.806) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.045 & 0.049 & 0.048 & 0.047 & 0.053 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.044** & 0.020 & 0.042** & 0.046** & -0.017 \\
& (0.019) & (0.022) & (0.019) & (0.022) & (0.027) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.084 & 0.087 & 0.086 & 0.085 & 0.091 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
\hfill
\begin{tabular}{lccccc} \hline \hline
& & & & & \\
\multicolumn{6}{c}{Subtable Name} \\ \hline
& & & & & \\
\multicolumn{6}{c}{Panel A} \\ \hline
& & & & & \\
LOSSFNK & 0.066 & -0.777 & 0.182 & 0.481 & 1.795 \\
& (0.467) & (0.856) & (0.486) & (0.549) & (1.263) \\
Observations & 204,520 & 204,520 & 204,520 & 204,520 & 204,520 \\
R-squared & 0.028 & 0.036 & 0.033 & 0.032 & 0.045 \\
& & & & & \\
\multicolumn{6}{c}{Panel B} \\ \hline
& & & & & \\
LOSSFNK & 0.045** & 0.044 & 0.044** & 0.044* & 0.048 \\
& (0.021) & (0.032) & (0.021) & (0.024) & (0.052) \\
Observations & 261,101 & 261,101 & 261,101 & 261,101 & 261,101 \\
R-squared & 0.062 & 0.069 & 0.065 & 0.065 & 0.075 \\
& & & & & \\ \hline
& & & & & \\
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \hline \hline
\end{tabular}
%}
%\end{threeparttable}
%\begin{tablenotes}
\tiny \raggedright
There should be some table notes here that go across all three subtables and not cutoff in the middle There should be some table notes here that go across all three subtables and not cutoff in the middle
%\end{tablenotes}
\end{table}
\end{landscape}
%%%%% Example 3 %%%%%
\begin{landscape}
\begin{table}
\caption{Caption of the Table}
\centering
%\begin{threeparttable}[]
\footnotesize %\setlength{\tabcolsep}{2.5pt}
%\resizebox{\linewidth}{!}{
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
\hfill
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
%}
%\end{threeparttable}
%\begin{tablenotes}
\scriptsize \raggedright
There should be some table notes here that go across all three subtables and not cutoff in the middle There should be some table notes here that go across all three subtables and not cutoff in the middle
%\end{tablenotes}
\end{table}
\end{landscape}
%%%%% Example 4 %%%%%
\begin{landscape}
\begin{table}
\caption{Caption of the Table}
%\begin{threeparttable}[]
\footnotesize %\setlength{\tabcolsep}{2.5pt}
%\resizebox{\linewidth}{!}{
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
\hfill
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
\bigskip
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
%}
%\end{threeparttable}
\hfill
\begin{minipage}{0.625\textwidth}
%\begin{tablenotes}
\scriptsize \raggedright
There should be some table notes here that go across all three subtables and not cutoff in the middle There should be some table notes here that go across all three subtables and not cutoff in the middle
%\end{tablenotes}
\end{minipage}
\end{table}
\end{landscape}
%%%%% Example 5 %%%%%
%\begin{landscape}
\begin{table}
\begin{threeparttable}[]
\caption{Caption of the Table}
\scriptsize
%\resizebox{\linewidth}{!}{
\begin{tabular}{lccccc}
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\toprule
\multicolumn{6}{c}{Subtable Name} \\ \midrule
\multicolumn{6}{c}{Panel A} \\ \midrule
LOSSFNK & 0.648* & 0.613 & 0.724** & 0.648* & 1.255 \\
& (0.337) & (0.580) & (0.360) & (0.364) & (1.053) \\
Observations & 508,578 & 508,578 & 508,578 & 508,578 & 508,578 \\
R-squared & 0.047 & 0.051 & 0.050 & 0.049 & 0.055 \\ \addlinespace
\multicolumn{6}{c}{Panel B} \\ \midrule
LOSSFNK & 0.050** & 0.048* & 0.051** & 0.047** & 0.021 \\
& (0.021) & (0.028) & (0.021) & (0.023) & (0.041) \\
Observations & 695,035 & 695,035 & 695,035 & 695,035 & 695,035 \\
R-squared & 0.080 & 0.083 & 0.082 & 0.081 & 0.086 \\
\midrule
Model 1 & Yes & Yes & Yes & Yes & Yes \\
Model 2 & Yes & Yes & Yes & Yes & Yes \\
Model 3 & & Yes & & & \\
Model 4 & & & Yes & & \\
Model 5 & & & & Yes & \\
Model 6 & & & & & Yes \\ \bottomrule
\end{tabular}
%}
\end{threeparttable}
\begin{tablenotes} \scriptsize
\item There should be some table notes here that go across all three subtables and not cutoff in the middle
\end{tablenotes}
\end{table}
%\end{landscape}
\end{document}
threeparttable
environment? Your code contains no\tnote
directives, while the only valid reason for using atablenotes
environment is to provide the footnote material to go with the\tnote
markers.resizebox
on your tabulars. This will give you a font size that is even smaller than tiny in your case.