Making flat rock with shadows

How do I make this rock more logical, in terms of brightness for each sides of it?

This is, by far I can do which is ridiculous

I manage to do it by the following:

• Create layer for each side/face of the rock


• Draw an irregular shape which is the side using Pen tool


• Create a base color which is brown


• Adjust the color brightness for each side in which it is facing

I will be needing different forms of this rock and I would like to do it in a very practical way. I think using 3D is much better but I have an insufficient knowledge on how to do it there using Photoshop.

Also, in which category of graphics design this is belong to? Is it flat?