Group Unit Kill Respawn

[JashDash]

In my game, only if ALL the creeps in one group die, should they re-spawn... but i just somehow cannot get the right method to do so. in my game 3 creeps spawn every time i kill only one of their creeps; not the entire group
[the creeps are spawned in a region on the map through a trigger]

Any help will be appreciated at the earliest possible. Thanks