Admiral Beckett Brass's triggered ability checks what happened earlier in the turn. It doesn't care whether Admiral Beckett Brass was on the battlefield when those Pirates dealt combat damage, whether those creatures are still on the battlefield, or whether they're still Pirates.

If one Pirate deals combat damage multiple times, most likely because it has double strike, it counts as only one of the three Pirates for Admiral Beckett Brass's triggered ability.

If more than one player was dealt combat damage by three Pirates, you target only one permanent. It may be controlled by either of those players.

The control-change effect of Admiral Beckett Brass's triggered ability lasts indefinitely. It doesn't wear off during the cleanup step or when you lose control of Admiral Beckett Brass.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well, and any effects that give the player control of permanents immediately end.

In a Two-Headed Giant game, an unblocked attacking creature deals its combat damage to one of the two players it's attacking. The attacking team chooses which player that is for each creature as combat damage is assigned. Combat damage dealt by Pirates your teammate controls will count when checking for Admiral Beckett Brass's ability.