Threesomes

Wikipedia

A threesome is a form of group sex involving three people of any gender combination.

Types
The level of sexual activity among the three members may vary greatly. Some threesomes may involve one who has a voyeur role, sometimes the invited member is there for enhancing the experience which does not involve having intercourse with the couple role (this is sometimes referred to as soft-swinging); other threesomes may involve some same sex contact, while others may actively involve all three membershttp://www.answers.com/topic/threesome-1.
Some people have made the argument that if the act does not involve sexual involvement between all three members that it is not a threesome at all and could be more accurately called a double team. This would be when two members of the threesome engage with one person but not actually with each other.
Group sex
One activity in this form of group sex is when sexual intercourse and/or anal sex are performed on the woman/man by two men in a double penetration. Another form of threesome is when a woman/man is given either intercourse and/or anal sex by one man (doggie style),http://www.myfirstthreesome.co.uk/node/117 while she/he gives the second man oral sex. Yet another example is a man and woman perform vaginal sex (from behind or reverse cowgirl), while a third person performs oral sex on the woman (clitoris) and/or on the man (testicles.)
Anecdotal evidence suggests that people have a wide range of reactions to being involved in a group sex situationhttp://sexperienceuk.channel4.com/videos/answering/have-you-ever-slept-with-more-than-one-person-at-the-same-time.

Ménage à trois
The term threesome should not be confused with the term ménage à trois, although both activities involve sex among three people.

Triad
It is most commonly used to describe a love relationship between three people where it can either take the form of a triangle (in which all three people have a love relationship with t read more at » http://en.wikipedia.org/wiki/Threesome