This article needs additional citations for verification. (February 2019)
In human sexuality, a threesome is commonly understood as "a sexual interaction between three people whereby at least one engages in physical sexual behaviour with both the other individuals". Though threesome most commonly refers to sexual activity involving three participants, it is also sometimes used to apply to a long-term domestic relationship, such as polyamory or a ménage à trois.
A threesome is a form of group sex which may occur in private situations, such as spontaneous sexual activity among three friends or in the context of casual sex or a hook up. Alternatively, it may take place in specific contexts or environments which allow for sex, such as swingers events, orgies or sex parties.
The people in a threesome may be of any gender and sexual orientation. Each participant may engage in any type of sex act with one or both of the others, such as vaginal, anal or oral sex or mutual masturbation. One or more of the participants may engage in autoerotic sexual activity, such as masturbation, possibly without physical contact with the other participants. It is a matter of subjective definition whether participation of a third person without physical contact constitutes a threesome and this sort of sexual activity might instead be interpreted as voyeurism or cuckolding. Triolism is a term that encompasses both threesomes and cuckoldry, although its usage across literature is inconsistent.
Threesomes are sometimes described using shorthand to refer to who was involved in the threesome e.g. MMF (male, male, female); FFM (female, female, male); MMM (male, male, male); FFF (female, female, female). 
Lucky Pierre is slang for a person performing both receptive and insertive anal and/or vaginal sex simultaneously during a threesome, being positioned between the two partners.
Academic research on threesomesEdit
The first major academic work to address threesomes specifically was published in 1988, called: Threesomes: Studies in Sex, Power, and Intimacy  by Arno Karlen. In the work, drawing mainly from interview data, Karlen outlined how threesomes were often viewed as qualitatively different to other forms of group sex. Other notable findings include that threesomes were often viewed by women as a safe way to explore their sexuality; they often consisted of a couple joined by a third person; and that the third person was not necessarily viewed or treated equally. Karlen also suggested that the societal view of threesomes cast those who engaged in them as radically different to other members of society:
There is a common tendency to think of people who have been in threesomes as alien beings. Like swingers, homosexuals and others who deviate from basic sexual norms, they seem to many to have entered another social, psychological, and moral sphere.
Research exploring rates of threesome engagement suggest that men have both higher levels of interest and participation in threesomes.  One study soliciting responses to a sex survey via a British newspaper found that 34% of 1,862 men, and 15% of 2,905 women had experience of a threesome.  From a nationally representative sample in the USA, 34.1% of men and 11.1% of women found a threesome to be at least somewhat appealing and 18% of men and 10% of women had engaged in one. 
Threesome sexual activity may take place in a number of sex positions; for example, the following:
Threesome scenes are featured in various films and TV series, including Summer Lovers (1982), Threesome (1994), American Psycho (2000), Zoolander (2001), Y Tu Mamá También (2001), Ken Park (2002), The Dreamers (2003), Kiss Me Again (2006), Shortbus (2006), Vicky Cristina Barcelona (2008), Shame (2011), 21 Jump Street (2012), On the Road (2012), Savages (2012), Spring Breakers (2012), Knock Knock (2015), Love (2015),You Me Her (2016–2020) and Holly Slept Over (2020).
- Scoats, Ryan (2019). UNDERSTANDING THREESOMES: gender, sex, and consensual non-monogamy. ROUTLEDGE. ISBN 978-0-429-84237-5. OCLC 1110711748.
- Thompson, Ashley E.; Byers, E. Sandra (April 2017). "Heterosexual Young Adults' Interest, Attitudes, and Experiences Related to Mixed-Gender, Multi-Person Sex". Archives of Sexual Behavior. 46 (3): 813–822. doi:10.1007/s10508-016-0699-1. ISSN 0004-0002. PMID 26943139. S2CID 39487837.
- Scoats, Ryan; Joseph, Lauren J; Anderson, Eric (February 2018). "'I don't mind watching him cum': Heterosexual men, threesomes, and the erosion of the one-time rule of homosexuality". Sexualities. 21 (1–2): 30–48. doi:10.1177/1363460716678562. ISSN 1363-4607. S2CID 151580022.
- Suzanne G. Frayser; Thomas J. Whitby (1995). Studies in Human Sexuality: A Selected Guide. Libraries Unlimited. pp. 340–341. ISBN 1563081318. Retrieved October 3, 2013.
- LEHMILLER, JUSTIN J. (2018). TELL ME WHAT YOU WANT: The Science of Sexual Desire and How It Can Help You Improve Your Sex Life. ROBINSON. ISBN 978-1-4721-4223-8. OCLC 1013584575.
- Jonason, Peter K.; Marks, Michael J. (March 2009). "Common vs. Uncommon Sexual Acts: Evidence for the Sexual Double Standard". Sex Roles. 60 (5–6): 357–365. doi:10.1007/s11199-008-9542-z. ISSN 0360-0025. S2CID 53548047.
- Ley, David J. (2009). Insatiable wives : women who stray and the men who love them. Lanham, Md.: Rowman & Littlefield. ISBN 978-1-4422-0030-2. OCLC 373474387.
- Claire Maniez (2012). Science and American Literature in the 20th and 21st Centuries: From Henry Adams to John Adams. Cambridge Scholars Publishing. p. 164. ISBN 978-1443835190.
- Karlen, Arno (1988). Threesomes : studies in sex, power, and intimacy. New York. ISBN 0-688-06536-8. OCLC 17104896.
- Thompson, Ashley E.; Cipriano, Allison E.; Kirkeby, Kimberley M.; Wilder, Delaney; Lehmiller, Justin J. (2020-11-11). "Exploring Variations in North American Adults' Attitudes, Interest, Experience, and Outcomes Related to Mixed-Gender Threesomes: A Replication and Extension". Archives of Sexual Behavior. 50 (4): 1433–1448. doi:10.1007/s10508-020-01829-1. ISSN 1573-2800. PMID 33175272. S2CID 226302189.
- Wilson, Glenn D. (1987-01-01). "Male-female differences in sexual activity, enjoyment and fantasies". Personality and Individual Differences. 8 (1): 125–127. doi:10.1016/0191-8869(87)90019-5. ISSN 0191-8869.
- Herbenick, Debby; Bowling, Jessamyn; Fu, Tsung-Chieh (Jane); Dodge, Brian; Guerra-Reyes, Lucia; Sanders, Stephanie (2017-07-20). "Sexual diversity in the United States: Results from a nationally representative probability sample of adult women and men". PLOS ONE. 12 (7): e0181198. Bibcode:2017PLoSO..1281198H. doi:10.1371/journal.pone.0181198. ISSN 1932-6203. PMC 5519052. PMID 28727762.
- American Psycho threesome scene. Retrieved 2015-10-24.
- Hoarde, Peter. "The 10 Most Memorable Threesome Scenes in Movies". Complex.com. Retrieved 29 September 2018.
|Look up threesome in Wiktionary, the free dictionary.|
Media related to Threesome at Wikimedia Commons