Boros Strike-Captain

Once a battalion ability has triggered, it doesn't matter how many creatures are still attacking when that ability resolves.

Similarly, once you attack with three or more creatures in a single turn, you can still play the exiled card that turn no matter what happens to those creatures.

If Boros Strike-Captain leaves the battlefield, the effect allowing you to play the exiled card if you attacked with three or more creatures continues to apply, even during later turns, as long as you attack with three or more creatures during those later turns.