We don't have a clear cut 3rd star but it's okay because we have 5 guys who are potentially our 3rd best player whereas other teams might have a 3rd star but then they suck after their 4th or 5th guy.
Here's a case for each of the 5 guys:
Rob: I mean, if you just look at the advanced stats, he actually has a case for being our best player, or at the worst our 2nd best player - definitely no worse than 3rd. Last season he was top 10 in the entire NBA in offensive rating, defensive rating, BPM, Win Shares Per 48 Minutes, TS%, FG%, PER, Rebounding % and blocks %.
If we knew for certain that he'd play in over 90% of our games and average 30+ mins a game then this wouldn't even be a thread. It would be obviously Rob. But that's a huge if. So that's why all of these other guys below have a chance to be the 3rd best player, simply because there's a decent chance that each of them sees much more court time (between games played and mins per game) than Rob.
Al: Since Rob's health is such a concern, that opens the door for Al to get some good playing time. If you look at most of the advanced stat categories for last season, Al was statistically the 4th best celtic after Tatum/Brown/Rob. Despite getting up there in age, Al posted really good numbers across the board in OKC, including a very strong BPM of +3.3 which is actually higher than his career average of +2.8. That +3.3 BPM was actually higher than Jaylen Brown's +2.5. Rob and Tatum were the only Celtics last yr with a higher BPM. Al averaged 14.2 PPG last yr which was also higher than his career average. Based on the stats and the footage we have available of Al from last season, Al is still a very good defender so when you combine that with his shooting, passing and craftiness on dribble drive moves, post moves he clearly still has a good all around game. He has a good case as the 4th best player on the team and due to Rob's health issues, if Al is able to see more court time than Rob then one could say that Al is our 3rd best player
Smart: Potentially an all-NBA defense guy, will probably be starting PG - has potential to be a true facilitator of the offense, responsible to distributing the ball, controlling the pace, getting 10-12 PPG along with 6.5 - 7.5 APG as well as being the ehart and soul of the team. Finding the open man, but also making teams pay if they help off of him too much. Guarding the other team's best perimeter player. That's his case.
Richardson: Simply put, it's the combination, defense and passing from the wing as well as being a steady veteran with playoff experience. Has as good a shot as anyone to be our 3rd leading scorer. Who is gonna be our 3rd leading scorer? Probably either Smart, Schroder or Richardson. I mean, maybe Horford but there's questions about whether he will play enough (games played as well as mins per game) to really be considered our 3rd leading scorer. So if we're looking at schroder vs smart vs richardson, this video (around the 6:00 mark) talks about how in terms of points per FG attempt, Richardson has performed better than both Smart and Schroder, and also Smart and schroder will be counted on to do more distributing than richardson will.
Next we have Richardson's defense. Defensively he has the potential to be our 3rd best defender after rob and smart. And if you look at defensive impact (how good they are defensively while also accounting for how much they actually play in terms of games played and mins per game), Richardson could end up being 2nd on that list - maybe even 1st since Smart has missed more time to injury over the past few yrs than richardson has. So if you have a guy who could be our 3rd best scorer and also out 1st or 2nd most impactful defender, while also being a guy who you really don't have to worry about much with injuries, he at least has a shot to be our 3rd best player
Schroder: here's his case. In terms of scoring volume, no other celtic on the current roster has shown the ability to score in the type of volume during the past 3 seasons that Schroder has. Also, no celtic on the current roster has put up the type of assist % numbers that schroder has put up. So the potential is there for him to be our best assist man and our 3rd best scorer, while also being our fastest player (i don't think there's really much doubt he's the fastest guy on the team), a higher energy and a good defender. You put all of that together and he's at least in the conversation for being our 3rd best player.