We suggest a tool to decompose 3D discrete objects into simpler parts. Decomposition is guided by object thickness and is based on region growing. Region growing starts from a set of seeds suitably identified in the innermost regions of the object. Our tool originates a decomposition into nearly convex and elongated parts. The decomposition is not affected remarkably by object rotation and has a limited computational cost. Since different recent imaging techniques give easier and cheaper access to 3D images, it is likely to believe that voxel-based decomposition tools, like the one here introduced, are of interest for future applications.