3 barrel definition.
We will talk about 3 barrel when a player who has the lead preflop (the player who did the most aggressive preflop action, probably a raise) bets on the flop, the turn and the river. The 3 barrel can be "value" because you have the nuts and want to get paid or bluff. We will then say that it is a 3 barrel bluff, a fearsome tool in aggressive games. You can make a 3 barrel bluff whether you are in position or not, the important thing is to have the lead.
A 3 barrel barrel in practice!
Let's take the example of a 3 barrel bluff. The player on the button opens preflop while the players before him have all folded. He has an ace and a queen in spades. The player on the small blind (SB) calls the bet and the big blind is folded. The flop falls 8-9-J with 2 spades. Here is an excellent flop for the original raiser (button). The player in the small blind position who speaks first to the flop decides to check. The player in the button position bets (1st barrel). The SB call. The turn is a brick, say a 2 of hearts. SB check and button bet (2nd barrel). Finally, the river is a brick, let's say a 2 of clubs and button decides to go for it all while betting his 3rd barrel, hoping to put down the small blind.