From Longman Dictionary of Contemporary Englishthreesomethree‧some /ˈθriːsəm/ noun [countable usually singular] informalHMN a group of three people or things
Examples from the Corpus
threesome• After dinner they had walked in the grounds as a threesome before Lee had suggested another trip to the pub.• One place or another, we always went as a threesome.• I didn't want to hurt either of them and I didn't see why we couldn't make a threesome.• He was also bored with Yolande and started bringing home girls, suggesting to Yolande that they try a threesome.• A player by himself may ask to join any threesome.• In every threesome one person is always the third.• We go round in this sort of threesome at playtime.• It is the threesome of snake, Eve, and Adam which, according to the Bible, makes for carnal knowledge. 