Given a binary matrix, deciding wether it can be decomposed into three
-convex matrices is an
-complete problem, whereas its decomposition into two
-convex matrices or two
-polyominoes can be performed in polynomial time. In this paper we give a polynomial time algorithm that decomposes a binary matrix into three
-polyominoes, if such a decomposition exists. These problems are motivated by the Intensity Modulated Radiation Therapy (IMRT).