Marvin, in particular, in not a normal role. I saw Michael Rupert do MARCH as a one act and FALSETTOS as a two act. He was brilliant in both, but he put as much energy into one act as he later did into two. (I.e., everything he had to give.) He was great in two acts; he was ferocious in one. Give him three acts to perform in one day and you simply disperse his energy that much more.
And we haven't even addressed the fact that Chip Zien was deemed right for Marvin in IN TROUSERS, yet was shifted to Mendel for MARCH. So it's reasonable to assume there are fundamental changes to the character between plays 1 and 2.
I think asking one actor to play all three acts in one day would be like asking an actress to play Evita three times in a day.
Of course, other performance configurations are possible. It doesn't have to be three in a day.