3 year old boy

I have a three year old that always seems to want to touch his penis and "make it big" he says. He would always do it just sitting on the couch or sitting beside me. Never in public. He stopped doing that after about a month of it. Now I just caught him sliding in the tub on his stomach and kept watching me to make sure I wasn't watching. When I pulled him up to get out I saw what was going on so I quickly got him dressed. Does anybody have a son that is or that did go through this? I've read it's all normal but it just gets under my skin and is something I am not comfortable in dealing with. I wish it would pass.