I've always thought about it but I wonder how well it would actually work. I do know of at least one other theatre that did them all, but I believe they did "In Trousers" separately from the other two at its own performances. I can't recall which theatre that was, perhaps it rings a bell for someone.
It would make for quite a long show to do them all in one evening. It would seem to make sense that Dr. Charlotte doubles Miss Goldberg and Cordelia doubles the High School Sweetheart and it adds no more people to the cast. But I just feel like "In Trousers" is so different in tone/style even though its ostensibly the same story. "March" and "Falsettoland" really do feel like two sides of the same coin - there's a consistency to the pieces that connects them in a way that "In Trousers", which is more of an abstract piece bouncing back and forth through time, doesn't really share.
There's also the question of which "In Trousers", since I know of at least three versions. There's the original with Chip Zien which is the recorded version, and has such anomalies as Marvin and Trina having two children among other things. Then there was the rewritten version which is licensed by Samuel French and was published in a compilation called "The Marvin Songs", which also had the one-act versions of "March" and "Falsettoland". But that version of "In Trousers" prominently includes "I'm Breaking Down", which was added to "March" with slightly altered lyrics. So do you do it in Act One or Act Two? Is it about her marriage falling apart, or her family?
Then there was yet a third version which combined those and appeared in the "Falsettos" published edition with Keith Haring cover art. I don't remember details of that version other than skimming it. I think it would be interesting for a company doing "Falsettos" to separately do "In Trousers" in rep, but as a full evening I wonder how satisfying it would be. I really think the style of the first wouldn't match up well with the other two and it wouldn't really add anything emotionally to the whole thing.