Table with non-standard multicolumns

Question

I want to typeset this table:

As you see, it's hard to use multicolumn here. I tried 2 consecutive tables:

\documentclass{article}
\usepackage[a5paper]{geometry}
\usepackage{tabularx}
\usepackage{lipsum}
\usepackage{array}
\setlength{\parindent}{0cm}
\begin{document}

\begin{tabularx}{\textwidth}{ | X | X | }
  \hline
  1 & 2\\
  \hline
\end{tabularx}
\begin{tabularx}{\textwidth}{ |m{20mm} | X|m{20mm}| }
  1 & 2 & 3\\
  \hline
\end{tabularx}
\end{document}

But there is a gap between them.

Any suggestions about typesetting the top table?

Answer 1

Something like this?

\documentclass{article}
\usepackage[a5paper]{geometry}
\usepackage{tabularx}
\usepackage{lipsum}
\usepackage{array}
\setlength{\parindent}{0cm}
\begin{document}
\begin{tabularx}{\textwidth}{ | X  X  X  X | }
  \hline
  \multicolumn{2}{|X|}{1} & \multicolumn{2}{X|}{2}\\
  \hline
  \multicolumn{1}{|m{20mm}|}{1} & \multicolumn{2}{X}{2} & \multicolumn{1}{|m{20mm}|}{3}\\
  \hline
\end{tabularx}\unskip
\end{document}