Here's a 3-way proposal. It's probably terrible -- forgive my first message board trade attempt:
Bulls get: Kemba Walker
Hawks get: Bulls 2021 1st, Grant Williams, Garrett Temple
Celtics get: Otto Porter Jr., John Collins, Kris Dunn. Celtics use Hayward TPE to fit in Porter, Kanter TPE to fit in Dunn, and Grant's salary to match Collins. This leaves them with a new TPE that equals Kemba's contract.
Bulls get an upgrade at their PG/SG position (Kemba replaces Temple who starts for them in a combo guard role). They're the 9th seed, so it lets them make a playoff push with a star vet under contract in the future. The Hawks get potentially a good pick for Collins, a PF in Grant who probably fits better next to someone like Capela, and a veteran combo guard to help take some of the burden off of Trey Young (especially with Rondo out). Celtics get Collins as well as terrific perimeter defender in Kris Dunn (when he returns from injury), and an even bigger TPE to use for the offseason/next year. Porter Jr. might prove as useful as Grant/Semi, but probably not much more than that. He's essentially an expiring that lets Chicago get involved in the deal.
Additionally, this would give the Celtics some luxury tax flexibility next offseason. This could be useful in getting Collins to resign for less than the max (my biggest concern is I don't think Collins at the max is a great idea) -- as he's a restricted free agent, other teams would be less likely to extend an offer sheet as they know the Celtics are capable of matching and would not be subject to the tax if they did.