\documentclass{article}
\usepackage{xcolor}
\usepackage{calc}
\usepackage{scrextend}
\usepackage{geometry}
\geometry{left=3cm,showframe}
\begin{document}
\begin{addmargin}[-2.9cm]{0pt}
\begin{minipage}{\linewidth}
\color{gray}
\rule{\linewidth}{5cm}
\end{minipage}\par\noindent
\begin{minipage}{2.9cm}
\color{gray}
\rule{\linewidth}{5cm}
\end{minipage}\hfill
\begin{minipage}{\textwidth-2pt\relax}
\color{gray}
\rule{\linewidth}{5cm}
\end{minipage}
\end{addmargin}
\end{document}