A game counter is all you need, along with the enemies needed to count towards the trigger amount and the multimanger you want to trigger. You need the following entities:
game_counter
name: killcount
target: trigger_mm
limit value: 3
At least three enemy NPCs
triggerTarget: killcount
triggerCondition: Death
multimanager
name: trigger_mm
That's it. When three of the enemies are killed, they each will trigger the game_counter (killcount) which will in turn trigger the multimanager (trigger_mm)