The middle person in a three-way, often erroneously reported as an all-male threesome. The person in the middle of all the action. It can be any combination of male and female partners, though "Lucky Pierre" is the term for the man in the middle.
This is not an exclusively homosexual act performed by three gentlemen. It can be performed by one lucky gentleman and two ladies, one of whom is equipped with a strap on dildo. It can be performed by two bisexual gentleman and a lady, it can even be performed by three ladies sharing two strap ons. The possibilities are numerous.