the third type of male body excretion.
to go number one is to pee. number 2 is to poop. so number three is to release semen from a dude's body.
a person can't "go number three" because, unlike a deuce, it can't just be pushed out of the body. Hence it has to be "put down".
"This video game is so awesome, it makes me want to put down a three."
"That guy Rick is such a loser, I bet he puts down a lot of threes,"