Tough to say until we see it in action. Isaiah is a great shooter and Fultz seems pretty good too. I think people underestimate how much that helped the Big Three, because it allows them to be good scorers even on weak side kick outs. The inability to shoot well is also what hurt other Big Three combos like Olajuwon, Barkley, and Pippen (or even when Payton played with Kobe, Shaq, and Malone) in LA.
Regardless of the fit, Ainge should still draft the best player available and figure out how to make it work later. Worse case we trade one of them.