"3" is a song by American recording artist Britney Spears, from her second greatest hits compilation The Singles Collection. It was released on September 29, 2009, as the lead single of the album by Jive Records. The song was produced by Max Martin and Shellback while Spears was on tour in Sweden. "3" is an uptempo electropop song, that features a heavy bassline and has influences of dance and rave. The song's lyrics refer to threesomes, while referencing Peter, Paul & Mary during the chorus.

The song received positive appreciation from contemporary critics, with some reviewers calling it a classic Spears song. "3" has also achieved commercial success by topping charts in the United States and Canada as well as reaching the top ten in many countries around the world, including Australia, Finland, Norway, Sweden and the United Kingdom. In the United States, the song debuted at number one in the Hot 100, becoming the first song to debut at the top position in over three years. The accompanying music video, directed by Diane Martel, featured Spears and her dancers in different black and white backgrounds. It received positive reviews from critics.

In July 2009, Spears went into the studio to work with Swedish songwriter and producer Max Martin, while she was in Stockholm, during the European leg of The Circus Starring Britney Spears. They had previously collaborated on earlier hits including "...Baby One More Time", "Oops!...I Did It Again", "Stronger" and "If U Seek Amy". Spears recorded the song with Martin and Shellback at Maratone Studios. Instrumental work was done by Martin and Shellback. The song was later mixed by Serban Ghenea at MixStar Studios in Virginia. "3" was announced as the lead single along with the release of The Singles Collection, and was sent to radio stations on September 29, 2009.

"3" is an uptempo electropop song, which opens with synths and vocal effects. The song is written in the key of F minor and is set in time signature of common time with a tempo of 138 beats per minute. Spears' vocal range spans from the high-tome of C ₄to the lower register of C ₅. The song has a basic sequence of Fm–E ♯ –B ♯ m–Fm as its chord progression. During the middle eight, the song slows down with synthetic strings and bass beats, and the section ends with a beat similar to a four-on-the floor, a rhythm pattern popularized in 1970's disco music. Throughout the song, Spears' vocals are autotuned that, according to the Daily Mail, gives the song "a futuristic, contemporary feel".

The song's lyrics is about threesomes. Unlike "If U Seek Amy", the song has no double entendre and it’s more straightforward lyrically. The innuendo in the lyrics such as "Merrier the more, triple fun that way" have been compared to the songs in Prince’s 1980's studio album Dirty Mind. The chorus has been compared to a playground chant and closes with an extended moan. During the second part of the chorus, there is a reference to the folk group Peter, Paul & Mary, that in the song is used as sexual slang for a threesome. Todd Martens of the Los Angeles Timescalled the reference "the cut's biggest oddity".

"3" received positive reviews from contemporary music critics. Monica Herrera of Billboardcommented 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". 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 Blackoutor Circus". On the printed issue, the magazine gave the song four stars and called it an "instant Britney classic". Clark Collis of Entertainment Weeklycalled it "a fembot voice, near-seizure-inducing dance-floor frenzy". While crediting the production for "elevating the song above typical dance products", the Los Angeles Timessaid that Spears sounded "sweetly generic". Bill Lamb of About.com said that although the lyrics are controversial, "the bottom line is this is another irresistibly catchy pop confection that beats out most everything else on pop radio today". He praised the chorus and the middle section and called the track "classic Britney". The song was also compared to Madonna’s "Celebration" (2009), since they "musically present nothing particularly new, but they do manage to encapsulate many of the elements that make the singer a star". Lamb later named it the most essential pop song of its time in the week of November 20, 2009. A.J. Mayers of MTV picked it as the eighth best song of 2009.

On October 8, 2009, "3" entered at number fourteen on the U.S. Bubbling Under Hot 100 and at number thirty-eight on the U.S. Pop Songs, becoming Spears' twenty-fourth career entry in the latest chart, most among all artists. On the following week, the song debuted at number one on the Hot 100, becoming Spears’ third number-one single in the United States and breaking many of the chart records. It made Spears the first artist in over three years to debut at the top position and the only non- American Idolartist in eleven years to do so. It was the sixteenth song in the chart history to debut at the top position and also the shortest title for a song reaching the top of the BillboardHot 100. The song also charted at number one on the Digital Songs, selling 255,000 downloads in its first week and having the highest one week total of any song, since Black Eyed Peas' "Boom Boom Pow" back in April 2009. "3" has sold over 1,600,000 copies in the United States as of February 2010. In Canada, the song debuted at number eighty-six on the Canadian Hot 100 on October 17, 2009 and rose up to number one the following week. It was certified two times platinum by the Canadian Recording Industry Association (CRIA) for sales over 80,000 copies.

The track debuted at number fifty on the Australian ARIA Singles Chart on February 16, 2009, with only two days of digital sales. The song peaked at number six on the week of October 26, 2009. It has since been certified platinum by the Australian Recording Industry Association (ARIA) with sales of over 70,000 copies. On October 12, 2009, the song debuted at number sixteen in New Zealand, making it her highest chart debut since "Womanizer". On its fifth week on the chart, "3" reached its peak position at number twelve.

On the UK Singles Chart, "3" debuted at number seven on November 16, 2009, becoming Spears' twentieth top ten hit on the chart. The track has achieved similar success throughout Europe reaching the top ten in Belgium (Wallonia), Czech Republic, Finland, Norway, Sweden as well as the top twenty in Austria, Belgium (Flanders) and Denmark.

The music video was filmed on October 5 and October 6, 2009 in Los Angeles, California, directed by Diane Martel and choreographed by Tone & Rich. Fashion stylist GK Reid worked closely with Spears to design the fashion. On October 15, 2009, images of the video were released. 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. When asked about the video, Diane Martel said,

"I think it's her next sexy video. It's a very simple video for her – I don't think you've seen her this much under a magnifying glass. Everything's really strong and playful. 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. [..] [Spears]' so sweet and funny and so normal and down to earth. It's really fun as a director to see her in front of the camera. She's so creative with this stuff. The video is very simple, it's very, very minimal. So it's her, her, her. And she's totally engaging every second. She really knows how to work with a camera and move. I was really impressed, and I have worked almost with every female artist in the business".

The video begins with Spears wearing a sparkly black dress at a dressing table, putting mascara and Circus Fantasy on. Then, it cuts to her holding her hair up and singing the first lines in front of a white background. There are also black and white scenes of her wearing a white leotard behind a foggy glass. She then appears wearing the leotard and glasses, while surrounded by four female dancers wearing black outfits, hanging from a raised barre. The word "sin" is censored and changed for the word "this" during the pre-chorus. In the chorus, she dances in front of a white wall with barcode-like light installations with six male dancers. There also scenes of her dancing provocatively with two male dancers. At the bridge, there are intercut close-ups of her that end with a smirk. The video concludes with Spears dancing with the two male dancers and cuts to a close-up of her looking into the camera.

Rolling Stonewriter Daniel Kreps compared the video with the music video of "Single Ladies" and complimented the choreography saying Spears had not danced with so much conviction since the In the Zoneera. Jocelyn Vena of MTV gave the video a positive review, stating that it's a "sexy, fast-paced clip" and that "there are a few moments in the video where Britney's personality shines" adding that "she smiles at the song's tongue-in-cheek lyrics, particularly when she finishes the song's bridge." Tanner Stransky of Entertainment Weeklycomplimented the video’s choice of wardrobe, but said that the video was "sparse", adding "there’s not much to the single besides trite, ridiculous advocation for a ménage à trois anyway, so why would the video go any deeper?". The Daily Mailsaid the video "features Britney writhing around in scantily-clad outfits with male dancers in the best way she knows how".

On December 15, 2009, the director's cut version of the video leaked online. 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 Collectionrelease".