Hmmmm. Right now three males, no females. Any behavior troubles at this point?
A friend of mine had two muscovy males who were friends - until she brought home four female runners. She has kept them separated due to the likelihood that the large heavily clawed muscovies would seriously injure (at least) the runners mating them. The drakes started fighting once females were introduced to the household.
If you introduce only two females, I can see that we now have competition between the males. A recommended number of females to males I have seen is minimum three to one. So if you are getting females, that would be nine.
Drakes can really abuse ducks and if there are only two females, I would be concerned for their safety.
Also, what breed are the drakes?