It is interesting debate. I don't think everyone is looking at it the same way. Break it down, our starters right now are:
Smart
Brown
Tatum
Horford
Williams
If you take out Brown, our starters are:
Smart
White
Tatum
Horford
Williams
If you take out Williams:
Smart
Brown
Tatum
Horford
Theis
Are you supposed to consider how good the replacement is when asking who is more important? In this case, Theis has already been a starter on an ECF team so it is hard to say he isn't sufficient even if less than Williams. Even a journeyman player like Theis can probably provide comparable defense and rebounding to Williams (albeit less rim protection but better perimeter defense). White is a pretty good back up for Brown but I don't think he can be the #2 on a ECF team as Brown would be.
Theis and Williams are different players. Theis does some things better in fact. Better mid range, better perimeter defense. Williams more blocks, more dunks. I feel that recognizing the fact that a journeyman level player like Theis can more or less replace Williams shows Williams is far less important than Brown.
Yes, it's difficult trying to just compare the 3 players because you have to consider the replacements, and also consider how the impact of removing either Tatum or Brown will affect whoever remains. Having two of them on the court makes it difficult to keep throwing double teams at one of them. It would probably impact the way they are defended, especially if the player they are replacing is not the same offensive threat.
The way I interpreted this thread was that it was trying to gauge which of the three players impacted winning the most, in light of Zach Lowe's article saying that Timelord has transformed the Cs defense because of the way that Ime is deploying him away from defending the main screen setter and allowing him to play a bigger role as the main helper on dribble penetration. Because Timelord doesn't have the same profile as the other two, but the way that Ime has used him has really impacted winning in the past couple of months. Which is why despite his injury history and sparse playing time Brad gave him a $48k 4 year contract. They know how much he impacts winning.
But again I see his role as more of a supplementary role, in that he makes everyone better, whether it's defensively, offensively as a rim roller as well as as a rebounder and passer finding cutters. Based on the stats and the eye test I still feel the two players we have that impact winning the most are Tatum and Brown, and they both make each other better. If you look at the net rating of our most used 2 man lineups, Tatum and Brown have the highest. That said, Timelord is right up there with either of them:

When it comes to 3 man lineups they are right at at the top of our most used lineups (I won't post yet another screenshot but will link the query I'm using if people want to mess around with it):
https://www.nba.com/stats/lineups/advanced/?Season=2021-22&SeasonType=Regular%20Season&TeamID=1610612738So my conclusion is that I would want all 3 of them, because the 3 of them together create a whole that is worth a lot more than the sum of the parts.