Wiktionary
troilism
alt. The practice of having sex involving three participants, i.e. threesomes. n. The practice of having sex involving three participants, i.e. threesomes.
Wikipedia
Troilism
Troilism (sometimes spelled triolism) refers to sexual activity in which three persons take part simultaneously.
Usage examples of "troilism".
Sadism and masochism, then, are usually unpopular topics for Rough Sexy Novels, while homosexuality, lesbianism, group sex, troilism, and most other bedroom activities are not only permissible but encouraged.
In England we have the consenting-adults package: no sex until you are twenty-one, no 'public' sex in clubs and bars, and no group-sex whatever (even troilism is indictable).