Yeah, I have.
Throughout high school, a close friend and I went steady (at different times) with a girl. About the time we graduated, we just hung together. A week of so after graduating, she began to say, "Let's do a threesome. This may be out last summer together." So we did one night at her family's cabin.
I was on the bottom, she climbed on top of me, and my friend took her from behind. What was it like? It was like having sex with both of them. I could feel his dick rubbing against mine, and I watched his face as he neared climax and then backed off. I exploded first, and he second maybe a millisecond behind.
Afterward we just sat on the couch wasted. For her, it was the ultimate experience, having sex with the two guys with whom she had spent many hours. For us, it was a more sobering experience, "Damn, I just had sex with a guy and my best friend. What does this mean?" A really big question for any teen.
We got talked into it a couple times more that summer, but the guys never watched the other's face. That helped.
jay