One of the best ways to learn 3 dimensional drawing is sculpting.

If you sculpt an object you really care about how it looks from all angles.  As a beginner you usually don’t think about the third dimension when you draw from a photo (f.e.). You try to replicate the 2d shapes you see but you do not (need to) understand why they are the way they are. And so you do not learn what makes a drawn subject feel 3d.

Sculpting helps to understand the construction of an object starting with primitive forms (cube, ball, cylinder, cone) and ending with complex details. If you sculpted a correct skull a few times and you can draw it in any angle from imagination.