A caster can summon a number of units per game equal to his number of actions. No matter about the type. Pay attention: since you need an action to control a summoned one, if you summon the third you'll probably let one out of control.
I always intended that three summonings were useful to put again on the field a slain unit you summoned before, not to place a bunch of creatures all together.
If you want more than a summoned unit, I suggest you to put a maximum of two per caster. If you need three of them, just place two casters.