during a three sum, it is the middle person in position penetrating one while getting fucked by the other. the top fucks the bottom top, the bottom top fucks the bottom.
“yo did you see what the top did with the bottom top that made the bottom quiver.”
A definition for sex. can be for straight or non straight partners. The top is the pleasure giver(a.k.a the one on the top) and the one on bottom is receiving. (aka the one on the bottom)