\definecolor{martinsblue}{rgb}{0,0.2,0.6}
\definecolor{martinsgreen}{rgb}{0.55,0.592,0.011}
\definecolor{martinsred}{rgb}{0.76,0,0.031}
\begin{tikzpicture}
\fill[martinsblue] (0,0) rectangle (6,3);
\fill[martinsred] (0,3) rectangle (3,6);
\fill[martinsgreen] (3,6) rectangle (6,3);
\draw[line width=3mm,white] (0,3) -- (6,3) (3,0)-- (3,6) (1.5,6) -- ++(0,-2) (4.5,6) -- ++(0,-0.3) (3,5.3) --++(3,0) ;
\end{tikzpicture}