"3D rectangle" is kind of vague. If he used terminologies like cube, tetrahedron, dodecahedron etc, I would be 100% percent sure about what he meant, but "3d rectangle"...

There is a chance that he might actually mean a rectangle with certain relief, and 3D alike, for example, a Tk button created like this:

$mw->Button(relief => "sunken");
[download]

is 3D alike.

If this is what he meant, what he really want is simply 2D shapes with "shadows". If this is the case, then much less maths is needed, and more importantly one needs a sharp sense of colors.