After downloading Unity and setting up and account with them, I receive periodic emails with helpful tips and tutorials for the beginners. More than likely, you do as well so this post may be redundant and of no use to you. The one I received today was on 2D Colliders. When I visited the links, I found the links for 3D Colliders. For your convenience, here is the direct link
https://unity3d.com/learn/tutorials/topics/physics/colliders?playlist=17120