What links the '60s pop groups The Byrds, Buffalo Springfield and The Hollies?

Crosby, Stills and and Nash / Crosby, Stills, Nash and Young

David Crosby was a founding member of The Byrds, Stephen Stills and Neil Young from Buffalo Springfield and lastly Graham Nash from the Hollies.

With former members of three important 1960s rock groups-the Byrds (Crosby), Buffalo Springfield (Stills and Young), and the Hollies (Nash)-Crosby, Stills & Nash was the epitome of the supergroup (a group formed by already revered performers) when it formed in 1968.

https://www.britannica.com/topic/Crosby-Stills-and-Nash-group

The key link between The Byrds, Buffalo Springfield, and The Hollies is that members from each group went on to form the legendary supergroup Crosby, Stills & Nash (and sometimes Young). David Crosby came from The Byrds, Stephen Stills was a founding member of Buffalo Springfield and Graham Nash co-founded The Hollies. Together, they formed Crosby, Stills & Nash, later joined by Neil Young (also from Buffalo Springfield), creating one of the most influential folk-rock supergroups of the late '60s and '70s.

https://www.goldradio.com/hall-of-fame/crosby-stills-nash-break-up-reunion-songs-members/