Two people are having sex. The third person is having oral sex performed on himself/herself by one of the people having sex.
Guy One: Yo, I totally took those two girls back home at the same time; it was awesome.
Guy two: You had a threesome?
Guy one: Not exactly. I only boned one of the chicks and ate the other one out at the same time. It was a Two and a Half-some.
Guy two: You are a legend!