Just add in the Levelblueprint a Variable(Boolean) for each trigger.
Now you write in every trigger that he should set his Variable in the Levelblueprint to true if he is active. After that an Custom Event should be called.
This Event Fires a Branch that checks if all 3 Vairables(all Triggers) are true and if they are the Branch fires the Matinee.
Flownodes:
Each Trigger: Event Overlapp –> Set “TriggerPassed”(True) –> Call Custom Event
Custom Event: Event –> Branch (Condition: All Triggers true?) –> Start Matinee