F3 is originated from a 2D Online game called
Maple Story.
In game players would press the F3 button to change the face of their character to a sarcastic face. The face can be used to annoy players after they did or say something stupid. Some players choose to use the term "F3" outside of game such as in everyday speech or
Messenger.
Messenger:
Friend: I just failed
my maths test.
Maple Player: LOL! F3
Friend: huh?
Maple Player: F3
In Game:
Player 1: Dude, let me in your party.
Player 2: No F3
Player 1: WTF man why are you so cocky online?
Player 2: F3
Player 1: Just wait 'till i see you tonight.
Player 2: ok F3.
Player 1: DSFsdlmf1#!%$!$sdlfmsdmfasdomg43
Real Life:
Player 1: You
acutally went there? F3
Player 2: Yeah it was F3
Player 1: Uber F3