This an alternative, made with tabularray
package. I include a table similar to yours and another one (better looking IMHO). In this case you probably don't need the siunitx
library, but just in case...
\documentclass{article}
\usepackage{lipsum} % dummy text
\usepackage{tabularray}
\UseTblrLibrary{booktabs,siunitx}
\begin{document}
\lipsum[1]
\begin{table}[ht]\centering
\caption{Original table with \texttt{tabularray}.}
\begin{tblr}
{% format
vlines,hlines,vline{5}={2}{-}{},
colspec={ccccccc},
cell{1}{1}={r=3,c=1}{}, % multirow
cell{1}{2}={r=1,c=6}{}, % multicolumn
cell{2}{2,5}={r=1,c=3}{}, % multicolumn
}% content
& 40 Mod\\
& method A && & method B \\
& 1 & 2 & 3 & 1 & 2 & 3 \\
0 & 0.68 & 0.81 & 0.73 & 0.67 & 0.86 & 0.74 \\
1 & 0.93 & 0.87 & 0.90 & 0.93 & 0.89 & 0.91 \\
2 & 0.98 & 0.92 & 0.94 & 0.98 & 0.91 & 0.94 \\
3 & 0.35 & 0.30 & 0.32 & 0.34 & 0.30 & 0.32
\end{tblr}
\end{table}
\lipsum[2]
\begin{table}[ht]\centering
\caption{Alternative table also with \texttt{tabularray}.}
\begin{tblr}
{% format
colspec={cS[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]},
cell{1}{1}={r=3,c=1}{}, % multirow
cell{1}{2}={r=1,c=6}{}, % multicolumn
cell{2}{2,5}={r=1,c=3}{}, % multicolumn
}% content
\toprule
& {{{40 Mod}}}\\
\cmidrule[lr]{2-7}
& {{{method A}}} & & & {{{method B}}} \\
\cmidrule[lr]{2-4} \cmidrule[lr]{5-7}
& {{{1}}} & {{{2}}} & {{{3}}}& {{{1}}} & {{{2}}} & {{{3}}} \\
\midrule
0 & 0.68 & 0.81 & 0.73 & 0.67 & 0.86 & 0.74 \\
1 & 0.93 & 0.87 & 0.90 & 0.93 & 0.89 & 0.91 \\
2 & 0.98 & 0.92 & 0.94 & 0.98 & 0.91 & 0.94 \\
3 & 0.35 & 0.30 & 0.32 & 0.34 & 0.30 & 0.32 \\
\bottomrule
\end{tblr}
\end{table}
\end{document}

\cline{2-7}
for this.