You can also add an adjustment layer (with nothing on it) below the 3D layer giving you grief – this way you can keep the bottom layer 3D too. What this does is render everything below the adjustment layer and puts it into a 2D buffer, and then renders the rest of the stuff on top in 3D. Its a great way to have full 3D but separate layers that might intersect.
Its also great for making shadows as graphic elements on 3D planes below the object “casting” the shadows when you don’t want to use 3D lights or if you are doing a perspective hack where any cast shadows wouldnt really angle correctly.