Multiple panel figure, with caption in one of the panels?

Question

I want to put three figures inside a multi-figure with a single caption. If you imagine a 2x2 table, I want to place the three figures in three of the table cells, and the caption in the remaining cell.

Each subfigure should have its own label, and small captions (like (a), (b), and (c), to number the figures inside the panel). Then inside the caption for the whole figure, I want to reference the subfigures, but without using the figure number (that is, just (a) instead of Fig.3(a)), but outside the panel, I'll reference the subfigures as Fig.3(a), etc.

For example,

\begin{figure}
\centering
    \begin{subfigure}{.4\textwidth}
        \centering
        \includegraphics[width=\linewidth]{fig_a.pdf}
        \caption{}\label{fig:fig_a}
    \end{subfigure} %
    \begin{subfigure}{.4\textwidth}
        \centering
        \includegraphics[width=\linewidth]{fig_b.pdf}
        \caption{}\label{fig:fig_b}
    \end{subfigure} %
    \begin{subfigure}{\textwidth}
        \centering
        \includegraphics[width=\linewidth]{fig_c.pdf}
        \caption{}\label{fig:fig_c}
    \end{subfigure}
\caption{Some general caption of all the figures. In (\subref{fig:fig_a}) you can see a green square....}
\end{figure}

But this puts two figures ((a) and (b)) side by side on the first row, then (c) alone on the second row, and the caption appears below. The second row has much space unused. The following is a picture of what I expect to get:

How can I do this?

Answer 1

This may give you some ideas.

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}
\begin{figure}
\centering

\begin{subfigure}[t]{.4\textwidth}
\centering
\includegraphics[width=\linewidth]{example-image-a.pdf}
        \caption{}\label{fig:fig_a}
\end{subfigure}
%
\begin{subfigure}[t]{.4\textwidth}
\centering
\includegraphics[width=\linewidth]{example-image-b.pdf}
\caption{}\label{fig:fig_b}
\end{subfigure}

\medskip

\begin{subfigure}[t]{.4\textwidth}
\centering
\vspace{0pt}% set the real top as the top
\includegraphics[width=\linewidth]{example-image-c.pdf}
\caption{}\label{fig:fig_c}
\end{subfigure}
%
\begin{minipage}[t]{.4\textwidth}
\caption{Some general caption of all the figures. In (\subref{fig:fig_a}) you can see a 
green square....}
\end{minipage}

\end{figure}

\end{document}