I've been running three-team trades with the Suns and Hawks too. I don't think this one works because the Hawks want to keep Hunter. Also, the Suns need a center ready to compete in the playoffs to replace Ayton. Collins is more of a forward. This is especially true with McGee leaving the Suns for the Mavs.
The trades I've messed with all would require either Timelord or Horford going to the Suns, which is something I'm not really interested in doing.
Something like:
Suns: Horford, Capella
Celtics: Collins, Saric into TPE
Hawks: Ayton, Craig, Crowder
I think this positions the Suns to be real contenders again this year (especially if they can't land Durant) even while losing Ayton. Paul-Booker-Bridges-Horford-Capella is arguably better than last year's team that had the best record in the NBA.
I think this gets the Hawks a reset around a new core of Young-Murry-Ayton-Hunter-Bogdan. They're very luke-warm on Collins and this trade lands them their Center of the future.
I think this gets the Celtics a young power forward to replace Horford. Saric could be a sneaky helpful rotation player in the regular season as well.
Still, with the acquisitions of Gallo and Brogdon, I think I'd rather run it back with Horford. Every contending team wants a player like him, and I'm not sure why the Celtics would trade that away unless they thought he could not be an impact player in the playoffs again next year.