"Keep up with me, comrades, or I cannot guarantee you a fair share of the fracas."

Boros Strike-Captain

Battalion — Whenever this creature and at least two other creatures attack, exile the top card of your library. During any turn you attacked with three or more creatures, you may play that card.

Ravnica: Clue Edition

Battalion — Whenever Boros Strike-Captain and at least two other creatures attack, exile the top card of your library. During any turn you attacked with three or more creatures, you may play that card.

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.

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.

The three attacking creatures don't have to be attacking the same player, planeswalker, or battle.

You pay all costs and follow all normal timing rules for a card played this way. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.