\documentclass{article}
\usepackage[demo]{graphicx}
\usepackage{subcaption}
\newdimen\figrasterwd
\figrasterwd\textwidth
\begin{document}
\begin{figure}[ht]
\centering
\parbox{\figrasterwd}{
\parbox{.6\figrasterwd}{%
\subcaptionbox{}{\includegraphics[width=\hsize, height=8cm]{imageA}}%'height=8cm' is needed for this example only and can be dropped when using it with actual images
}
\hskip1em
\parbox{.3\figrasterwd}{%
\subcaptionbox{}{\includegraphics[width=\hsize]{imageB}}
\vskip1em
\subcaptionbox{}{\includegraphics[width=\hsize]{imageC}}
}
}
\end{figure}
\end{document}