LOL here is the official definition:
Threesome: An act of group sex involving three people. Triadic group sex regardless of gender: two women and one man, two men copulating the vagina and rectum of a woman, three men engaged in a chain or train of anal intercourse with the middle man being passive and active at the same time, or two men, one from the front, the other from the rear simultaneously penetrating the anus of a third man. The first two instances could be a domestic arrangements where a married couple share their bed with a third party, a stranger or the lover of the husband or wife.
Bibliography: A. Karlen,
Threesomes: Studies in Sex, Power, and Intimacy (NY: Beechtree Books, 198
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