I think most or all of the civs are playable. I don't know how balanced it would be with 3 players. For example, if it were French, Scots and English, the English would likely be at a disadvantage. I have no particular reason for wanting to play a Hundred Years War scenario other than I was able to guess that it would work reasonably well with 2 players, and I felt I should make a suggestion of a scenario.
I happen to have bought ToT today (I was considering it for a while), but I don't know when it will arrive; it may be a couple weeks (or only a few days). If you know of an MGE scenario for a small number of players, we could play that and wait to start this as ToT game.