Ah, this is tough one. I presently don't do anything special with sorting faces based on average depth before rendering, so objects with transparency / alpha are hard to deal with, because they may render out of the order needed by transparency (canonical 3d / transparency issue.)
If you are just rendering a cube though, depending on the orientation, you may just want to try 3DShape w/ the textures you want, if I remember correctly, for the simple cube case, C3 does sort as best as it can.