A threesome is when three people are involved in a sexual encounter at the same time. Also known as a menage a trois, a threesome is a form of group sex and can be classified as one of four main types: FFM (female-female-male), MMF (male-male-female), all female, or all male. Each participant may engage the others in a variety of sex acts such as : oral sex, mutual masturbation, vaginal, or anal sex; and can involve double penetration.