Battalion — Whenever this creature and at least two other creatures attack, this creature gains double strike until end of turn. (It deals both first-strike and regular combat damage.)
The three attacking creatures don't have to be attacking the same player, planeswalker, or battle.
Once a battalion ability has triggered, it doesn't matter how many creatures are still attacking when that ability resolves.
The effects of battalion abilities vary from card to card. Read each card carefully.