Ok, I think I found a legitimate contender.
Jizzy Pearl formed a band called Love/Hate in the 80s, becomes an established artist.
So established artist who joined 3 established bands.
Darryl Jones is a similar situation. Became known playing bass in Miles Davis’ band, recorded and toured with Sting right after the Police broke up, and has been touring and recording with The Rolling Stones since 1993 (only 4 years less than Bill Wyman), yet has never been made an official member. Does he count?
Ron Wood made a name for himself in the Jeff Beck Group; then The Faces, Rod Stewart’s band; and now, of course, The Rolling Stones.
John5 was a session guitarist and in a bunch of bands with middling success before joining David Lee Roth Band, then joined Marilyn Manson, then joined Rob Zombie which is debatable if its a solo act or a band (i consider it a band).
Chris Pennie was the drummer in Dillinger Escape Plan, but then later joined Coheed & Cambria for a few years in between Josh Eppard’s stints with the band.
John Hawken. His history is convoluted, but after first coming to prominence with the Nashville Teens, he became a member of the UK group Third World War for their second album. He then became a member of the Strawbs, replacing Rick Wakeman. At one point, he also was touring with Spooky Tooth.
Jeff “Skunk” Baxter fits the bill. He joined the band Ultimate Spinach for their 3rd album, and became somewhat known in rock circles. He then joined the Holy Modal Rounders, before being a founding member of Steely Dan. After that he joined the well established Doobie Brothers from 74-79. He later played with pretty much everyone but I don’t know if he qualifies as a band member, session player, or collaborator for all of those acts.
But joining the already established Holy Modal Rounders and The Doobie Brothers should qualify.
Clapton: Started in The Roosters, then joined The Yardbirds, then Bluesbreakers, a bunch of stuff that doesn’t count in this thread, then joined Delaney & Bonnie.
Jim Gordon. It’s hard to figure out where he was a member of a group or just a backing musician, but he did join Traffic and Frank Zappa after they were established, and played in the studio with Steely Dan.
Rudy sarzo was in Quiet Riot, Dio, Ozzy Osbourne, and Whitesnake. Vivian Campbell got his start with Ronnie James Dio and then later played with Whitesnake and Def Leppard.
And he was also drummer for Skin Yard (sort of famous for being one of the original grunge bands) before Soundgarden.
Speaking of Pearl Jam:
Stone Gossard and Jeff Ament were both in Green River and Mother Love Bone.
Bruce Kulick. I don’t think he ever had a glorious solo career, but has been well known since around 1968. He has joined a number of bands that were well known, but past their sell-by date, including Good Rats, Kiss and Grand Funk Railroad (his current billet).
All these posts about the Doobie Brothers and no mention of Michael McDonald…
Yeah, but they were founding members of both. Also, Mother Love Bone was a “super group” (a one-off band composed exclusively of previously-established musicians, like Cream or the Traveling Wilburys) and a little outside this thread’s purview, known musicians who joined multiple already-established bands.
Jesse Tobias. His band Mother Tongue was well-known in LA, don’t know if they had a recording contract, but from there he went through three already-established bands, Red Hot Chili Peppers, Alanis Morrisette and Morrissey.