David Crosby (formerly of Byrds), Stephen Stills (formerly of Buffalo Springfield), and Graham Nash (formerly of the Hollies) formed the creatively named Crosby, Stills & Nash supergroup in 1968. There were no formal ties between the three; they had just played together in non-formal settings and were wrapping up their involvement in their previous bands around the same time. Starting in 1969, Neil Young (who knew Stills from Buffalo Springfield) was in and out of the group. This supergroup is the first band to have all members inducted into the Rock and Roll Hall of Fame twice, true supergroup status!
Despite their popularity and success, there was frequent transition between being a trio and being a quartet due to conflicting dynamics within the group. They started as CSN in 1968, became CSNY in 1969, and then went on a hiatus in 1970. Until 1973 everyone was working solo, and then CSNY 2.0 arose in 1973. By 1976 they were down to CSN. Young made his final stint in the band starting in 1988 but wasn’t always part of the touring, especially in the 2010s.