Three-act theater is actually not that dominant historically, with the five-act structure being as present, if not much.
But basically, you have to deal with Renaissance italian theater (and the huge influence it had on European theater) and use of Aristotle (more systematized than actually present) on its regulation (not only three-act, but as well unity rules).
Would these being less present, an European theater more close to a mix of late medieval theater, Elizabethan theater or Corneille's plays may appear : as in existence of rules but not systematical ones and less coercive in order to paint something less about emotional plausibility than emotional sublimation.
Uh, what do you even mean? A lot of Shakespeare Plays are 4/5 Acts, so it's not like it was ever homogonized. I don't think Greek or Roman theatre even had the concept of acts.
Elizabethan theater is a bit of an exception in the XVI/XVIIth centuries : everywhere in the continent classical theater used more coercitive and homogenized rules (look at classical theater unities).
As for Greeks/Romans, Aristotle already theorized an approach on acts (prologue, parados, episode, stasimon, exodos), while Horatius is probably more systematizing about how many acts there should be in a play (Five, for him).