باید با این حل بشه
\documentclass{article}
\usepackage{amsmath}
\usepackage[demo]{graphicx}
\usepackage{fancyref}
\usepackage{subcaption}
\begin{document}
\begin{figure}
\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=.95\textwidth]{wu8_wedges}
\caption{With wedges.}
\label{fig:wedges}
\end{subfigure}%
\begin{subfigure}{0.5\textwidth}
\begin{subfigure}{\textwidth}
\renewcommand\thesubfigure{\alph{subfigure}1}
\centering
\includegraphics[width=.95\textwidth]{wu8_flat1}
\caption{}
\label{fig:flat1}
\end{subfigure}
\begin{subfigure}{\textwidth}
\addtocounter{subfigure}{-1}
\renewcommand\thesubfigure{\alph{subfigure}2}
\centering
\includegraphics[width=.95\textwidth]{wu8_flat2}
\caption{}
\label{fig:flat2}
\end{subfigure}
\addtocounter{subfigure}{-1}
\caption{Without wedges}
\label{fig:flat}
\end{subfigure}
\caption{Interactions between the railcar and the man's feet.}
\label{fig:interactions}
\end{figure}
\end{document}