I used to see a lovely young woman who would host incall in a nondescript motel room. We always had one heck of a good time. However, she would also bring her dog with her. I don't know much about dog breeds, but this one was quite small, had long hair, was a male, and was definitely not neutered.
So, one time well in to the session, we (that is, me and the woman) are going at it hot and heavy "doggy style." She's on all fours at the end of the bed and I'm standing on the floor. Well, as you can imagine, it was in-out, back-forth, moaning, sweating, just a great ol' time.
Well, while all this activity was going on, her little dog was getting quite excited. Dogs are generally smart, and I'm pretty sure this one could figure out exactly what me and his owner were engaging in. We obviously got the dog's hormones raging, so he did what any excited male dog will do when there is not a female dog in the immediate vicinity. He went straight for a human leg! In this case mine!!
That's right. He ran up to one of my legs (I can't remember exactly if it was the left or right), wrapped his front paws/legs around it, and started humping that leg, almost as vigorous as I was humping her! I kind of screamed, then broke from my position behind her, and fell on to the bed. She turned around and saw her dog still attached (and still going at) my leg. At that point I managed to kick the dog off of my leg. Both of us (that is, me and the woman) then started laughing hysterically and we must have laughed together for 5 full minutes!
And that's my story, my first (and only) 3-way doggy style.