I'm writing my bachelors thesis, and I have som trouble positioning my subfigures. I have three figures in different sizes, and I would like to have an output like this:

I tried typing the following code:
\begin{figure}[H]
\centering
\begin{tabular}{cc}
\multirow{2}{*}{%
\begin{subfigure}[b]{.38\textwidth}
\centering
\includegraphics[trim=8cm 8.5cm 8cm 4cm,clip,width=.5\textwidth]{fig/mast01.eps}
\end{subfigure}} &%
\begin{subfigure}{.58\textwidth}
\centering
\includegraphics[trim=13cm 11cm 2.5cm 12cm,clip,width=.6\textwidth]{fig/mast01.eps}
\end{subfigure} \\ &%
\begin{subfigure}{.58\textwidth}
\centering
\includegraphics[trim=7cm 16cm 2.5cm 2cm,clip,width=.6\textwidth]{fig/mast02.eps}
\end{subfigure} \\%
\end{tabular}
\caption{Teknisk tegning af gittermasten}
\label{fig:gittermast}
\end{figure}
... but the output looks horrible.
How can I do this?

\documentclass{...}, the required\usepackage's,\begin{document}, and\end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Henri Menke Jan 11 '15 at 16:56