If you really want a 3D effect, place your black strips on just 2 adjoining sides, eg right and bottom only. This makes it look like the light is coming from the upper left.
Just try to picture the light source you want to achieve, and put the dark shadows opposite that. Try to have it coming from one corner, not one side. If you look at real shadows, you will see that they are not black, but a darker shade of the original colour.