premise: Beastie Boys were an American hip hop group from New York City, formed in 1981. For the majority of their career, the group consisted of Michael "Mike D" Diamond (vocals, drums), Adam "MCA" Yauch (vocals, bass) and Adam "Ad-Rock" Horovitz (vocals, guitar).
hypothesis: Mike D, MCA, and Ad-Rock were the only founders of the Beastie Boys.
reason: "For the majority of their career" doesn't tell us when - just that those 3 awesome musicians were the band was most of the time. We don't know who formed the band from this statement. (*coughcough Kate Schellenbach*)