The vernacular arose when Dean sarcastically labeled the three of them in episode 5x13:
Dean: This is it.
Sam: This is what?
Dean: Team Free Will. One ex-blood junkie, one dropout with 6 bucks to his name, and Mr. Comatose over there. Awesome.
Fandom quickly picked up the phrase as shorthand to refer to the three characters as a unit.
Fan 2: yeah and they kicked ass
Guy2 : "I've got a little TFW going on. Wanna pick up some midgets and get a group orgy going?"
Guy1 : "Those sound like some shenanigans that I would be down for."