When three people have sex at once. See menage a trois
Three people, two of one sex and one of the other in a swinging encounter. mmf or ffm
three people having sex at once.
Also known as menage-a-trois. A sexual encounter between three people.
Three people, two of one sex and one of the other in a swinging encounter. Not the same as a "menage a' trois" in that the latter may involve emotional involvement and a continuing relationship.
A threesome is the term which describes a sexual act involving three people, sometimes also referred to as a ménage à trois.