“3” by Britney Spears is featured in Britney 2.0, the second episode of Season Four. It is a stripped down acoustic version, unlike the original, which is very upbeat. It is sung by Joe (Samuel Larsen), Sam (Chord Overstreet), and Tina (Jenna Ushkowitz).
The song is the shortest song title on Glee and the only title to feature one number.

Tina, Joe and Sam perform this song for Brittany, who at the end of the song gets up to try to shave her head.