The doctor and his wife.

 They were caravanners touring around. We talked and arranged to meet. They were ready to have sex as soon as they arrived. He had a silk robe on. She had some hot lingerie under her clothes. This turned out to be the first time I had sex with three people in one encounter. I fucked his wife, fucked my wife and, at his wife's bidding, played with his cock as she fucked my wife with the double dildo. Would I do it again? Yes, under the right circumstances.

Three partners in a night?

 If two couples get together it is possible to have three sex partners in a night. A bi woman can have both the guys as well as the other woman. A bi guy can have sex with both the women and the other guy. Have I done it - yes, I have. Would I do it again - certainly!