We used to have a saying that "he's hornier than a three balled tom cat". Well you really are one. Have fun with it. Show them to her with pride and let her feel them to prove they are in there. I bet she'll think it's great.
I'd be thrilled to meet a girl with two clits. Why wouldn't a girl like to have a guy with three balls? It would certainly make for good conversation with her friends. It might get you more fun than you can handle if they want to check you out to see if she's lieing to them.
When I was about 6, my playmates who were all girls with circumcised brothers, were very interested in that foreskin I had that could be pulled back to reveal the same head their brothers had. Then it could be rolled forward to cover it up again. We soon discovered if they touched it to roll it back, my penis did a cute standup trick. They just had to tell all their girl type friends and cousins about that. When they came to visit they wanted to see it too. I enjoyed the h*ll out of all those girls playing with my toy. That's when I discovered the magic of a girls touch.
So I don't think your extra ball should be a problem, just the opposite.
If you are worried about undressing in front of a girl because she might say something about your third testicle, then you shouldn't be having sex with her.
Everyone is different. For you, this is a gift, look at it that way. It makes you special. Like njoynlife, I think of it as an advantage, not a negative.
A GP won't do anything.