At some point, the Celtics are going to need to decide who their two primary guards are going to be. There's room for only 2 of Thomas, Smart, and Bradley... in part because lineups with all three of them on the floor are generally ineffective (net -6.8 points/100 possessions in 262 minutes), particularly on defense.
At the same time, I think that having Smart log so much time at forward (and so little at the point) has slowed his offensive development, because PG is really the only position where he has flashed the potential to be a plus offensively. His shooting continues to be awful (as is his willingness to brick away from deep in any situation), but court vision, passing, and the ability to create offense (for himself or teammates) by posting up guards are all areas where the potential is there but the skills are raw and underdeveloped.
Bradley is the best outside shooter on a team that isn't loaded with shooting and also the best one-on-one defender of smaller guards.
Thomas is Thomas - one of the league's best offensive players, either on or off the ball, but always limited defensively by his lack of size. He'll also command the biggest bucks when extension time rolls around.
The Celtics also have some decent options at guard behind this group: Rozier looks like he can at least be a good backup guard, Jaylen Brown looks like he will be able to play minutes there, and even Demetrius Jackson looks like a good developmental project.
But some of their recent success has come from limiting IT/Bradley/Smart minutes. Each of them is an NBA starting caliber player but as a unit they are a mess. Whatever the next step forward for this franchise is, it probably involves moving one of these 3 guards.
I think Bradley is the guy I would move, much as I like him as a player.