Finding a third team is hard. The Celtics would basically need someone who is willing to part with a young, cheap big who could potentially play some minutes in a playoff rotation so that they can afford to get rid of Al or Rob. You also need them to have either cap space or a large expiring big enough to take on Brogdon or Horford. That's a tough ask.
Something like this works in the trade machine:
Celtics get: Jrue Holiday, Nick Richards
Blazers get: Gordon Hayward (expiring with value), Al Horford (has some value), Payton Pritchard
Hornets get: Brogdon, Kornet
You probably need to ship picks to both the Blazers and the Hornets to make it worth their while. Not really worth it probably.
Someone more creative will probably figure something better out, but I'm having a tough time finding a third partner who will play ball with us.