This sound is from Sexy Moans Soundboard (NSFW)

"3" is a song by American singer Britney Spears from her second greatest hits album, The Singles Collection (2009) The song was a commercial success, topping the charts in the United States and Canada, as well as reaching the top ten in many countries around the world.
See also: Music in pornography, Justify My Love, Tunnel Vision (Justin Timberlake song), and Justin Timberlake videography.