Threesum
Sexual Intercourse involving a male and 2 women.
intercourse involving 2 males and 1 women is a train.
intercourse involving 2 males and 1 women is a train.
Example:
The crowd began to advance upon the Threesum, and Omally was in the vanguard. By then the Threesum was already past its prime.
The crowd began to advance upon the Threesum, and Omally was in the vanguard. By then the Threesum was already past its prime.