How to use Masks with Element layers
It is not recommened to apply layer masks directly to the Element 3D layer due to it changing the shape of the layer and size. What you can do is create a new Solid above the Element layer and add a mask to the new solid. Then set the Element layer to one of the Track Matte modes. If you don't see the Track Matte options you may need to hit the Toggle Switches/Modes button at the bottom of the timeline to cycle through the layouts till you see the TrkMat drop-down. The Element layer will then use the layer mask above as a matte.