You already have had several expert opinions. Any doctor can see a wart, and it sounds as though none of the three who examined you saw anything at all. Rows of bumps around the penis head are called pearly penile papules and are normal. If you need more reassurance, see a dermatologist again, but when you hear that you don't, then it's time to move on and deal with what's really going on here--guilt and anxiety.
Good luck.
Dr. Rockoff