"3" is a song by American singer Britney Spears from her second greatest hits album, The Singles Collection (2009).

"3" is an uptempo electropop song that features a heavy bassline and synthesizers, and lyrics that talk about threesomes, while referencing American folk-singing trio Peter, Paul and Mary during the chorus as sexual slang.

"3" received positive reception from contemporary music critics, with some reviewers calling it a classic Spears song.

An accompanying music video for "3", directed by Diane Martel, features Spears and her dancers in front of different black and white backgrounds.

They had previously collaborated on earlier hits including "...Baby One More Time", "Oops!...I Did It Again", "Stronger" and "If U Seek Amy".

[3] On September 23, 2009, Jive Records officially announced the release of a greatest hits titled The Singles Collection through Spears's official website, in celebration of Spears's ten-year anniversary in the music industry, also announcing the release of "3" as the lead single from the album.

The innuendo in the lyrics such as "Merrier the more, triple fun that way" have been compared to the songs in Prince's 1980 studio album Dirty Mind.

Monica Herrera of Billboard commented that the song "builds to a climax of wildly pulsing bass that summons fans to the dancefloor" and added that "[it] will be another notch in this pop provocateur's belt".

[15] On its online edition, Rolling Stone's writer Daniel Kreps praised the song for its up-tempo melody and racy lyrics, while comparing it to Flo Rida's recent work, saying that "3" "is more of a surefire dance-floor stomper than anything Brit loaded onto Blackout or Circus".

[14] In his review of The Singles Collection, Stephen Thomas Erlewine of AllMusic selected "3" as one of the "track picks" and commented, "[it is] much better than any of the three new cuts on My Prerogative".

The song was also compared to Madonna's "Celebration", since they "musically present nothing particularly new, but they do manage to encapsulate many of the elements that make the singer a star".

[20] The following week, the song debuted at number one on the Billboard Hot 100, becoming Spears' third number-one single in the United States and breaking many of the chart records.

[35] A countdown, including images and video previews, was announced on her official website and the release date was revealed to be on October 30, 2009.

We collaborated on the wardrobe and had meetings about hair and makeup people, we talked a lot on the phone and met Tone and Rich — two of the best choreographers.

Kreps commented that the new version "offers up a slightly more NSFW edit of Martel's dance-heavy vid that seems to find a better cadence with the music, as opposed to the original version, which was essentially filmed and edited in the matter of days to coincide with The Singles Collection release".

The setlist of the show consisted on three songs from her seventh studio album, Femme Fatale, including "Hold It Against Me", "Big Fat Bass" and "Till the World Ends".

After "Up n' Down", Spears put on a white fedora and trench coat and performed the song with her female dancers, dressed in a similar style.

Shirley Halperin of The Hollywood Reporter named it one of the best performances of the show along with "Piece of Me" and "Don't Let Me Be the Last to Know", stating that "ironically enough, [they] were the ones with fewest frills.

"[46] Rick Florino of Artistdirect said "'3' gave way to a massive sing-a-long" and added that it "[spoke] right to the noir-ish connotations of the Femme Fatale tour.

"[47] Spears performed "3" as the third song on her Las Vegas residency, Britney: Piece of Me, as part of the original setlist.

After a brief talk with the audience, the singer began a performance of the song, jumping into a triangular cage which was wheeled around by her dancers across the stage.

For Billboard's Keith Caulfield, the song "served as the sexiest jungle gym in Vegas, as Spears and her dancers undulated their way across and around the bar".