"Everything is permitted? I like the sound of that."

Admiral Beckett Brass

Other Pirates you control get +1/+1.
At the beginning of your end step, gain control of target nonland permanent controlled by a player who was dealt combat damage by three or more Pirates this turn.

Secret Lair Drop

In her friends she inspires loyalty. In her enemies, mutiny.

"You and your ship will make a fine addition to my fleet."

The Lost Caverns of Ixalan Commander

The List

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 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.

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.

In a Two-Headed Giant game, combat damage dealt by Pirates your teammate controls will count when checking for Admiral Beckett Brass's ability.

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.

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.