Best way to do this would probably be to use a game_counter entity. That will open the door as soon as you've killed the set number of enemies.
game_counter
name: lock
target: door1
Initial value: 0
Limit value: 3 (or however many enemies you want)
func_door
name: door1
Enemies (whichever monster_entity enemies you want to use)
TriggerTarget: lock
TriggerCondition: Death