A code phrase between males. "Part 3" can be anything sexual you want it to be. It can be making out with a girl, throwing a bone at a girl, or anything you want with a girl. It is usually sexual intercourse in which the reproductive parts are put into action. It is used by males around their girlfriends to trick them into agreeing for a "part 3". Then, you would brag to your guy friends by saying that you've had a "part 3."
This follows part 1 and part 2.
Brian: "Hey babe, you remember our part 3 last night?"
Jenna: "Huh? Sure."
Wesley: "Wanna part 3?"
Meghan: "Uhh... sure?"
Brian: "Dudes! We hella part 3'd!"
Wesley and Brian: "Ooooh shit! DAMN! PART 3! PART 3!"
It's when you start "talking" with a girl. It's the beginning of "part x.xx", which is similar to the "bases" for relationships. The closer you get with the girl, the higher the number. If you start going out with the girl, that turns into part 2. One night stands can be as high as part 1.99 since you're not actually going out with the girl.
Brian: I heard that "G" got up to part 1.99 with some guy. Wtheo!
Wes: Yeah. Dude, she's not going out with that guy, but they got pretty far.
Brian: I got to part 2 already. Even up to part 2.8, woohoo.
Brian: I haven't gotten to part 1 because of how long I've been in a relationship. Yee.