A three-minute pop song is a cliché that describes the archetype of popular music, based on the average running-length of a typical single. The root of the "three-minute" length is likely derived from the original format of 78 rpm-speed phonograph records; at about 3 to 5 minutes per side, it's just long enough for the recording of a complete song.
References
Three-minute pop song Wikipedia(Text) CC BY-SA