The stamina draining is a cool idea, but it would be fun if they regenerated a bit so you wouldn't get messed up by having no available stamina left. Maybe I'm just playing too fast, but I never saw it regenerate except between waves.
It does kinda feel like I'm just one person who's switching their weapon though, as opposed to three people working together. Could there be some way to help differentiate them through movement? Like maybe the wizard teleports, the knight is slow cuz armor, etc.?