With 3 vs 3 we'd have to disappoint two people, though Brammer is starting another campaign game.
As to dividing forces: I agree that dividing them between 2 or 3 players works better than 4.
One option would be to have the CinC on each team handle the forces in the Netherlands, so the one player can focus on attacking/defending Belgium and one on attacking/defending France.
Giving the Allied player who controls Group of Armies 2 control over (most of) 9th Army might also be practical, as it allows the player who controls the rest of Group of Armies 1 and 7th Army to focus on Belgium.
First, let's wait on the scenario notes and let the other interested players chime in.