A 3-bet is the third bet in a betting round, and the conventional definition (and purpose) is different pre- and post-flop. The mechanics can be confusing for new players because the first ‘bet’ pre-flop in both Hold’em and Omaha is actually the (obligatory) Big Blind, making the initial raise the second bet (2-bet), and a subsequent (re)raise the third bet. Note that calls (‘limping in’) are not bets.
Here’s an example from pre-flop play: It’s folded around to the Button, who raises, after which the Small Blind re-raises – this is the 3-bet. Another raise would be a 4-bet…
Most players are familiar with – if not necessarily experts at – the concept of the 3-bet, usually in the context of pre-flop play. Indeed, once more associated with the Fixed Limit format, three-betting is now a crucial part of the modern NLH game, particularly in online poker. Whether to 3-bet is a key part of poker strategy, as is considering an opponent’s 3-betting range.
Post-flop 3-betting is different in more ways than one. First, at this betting juncture there is no Big Blind to represent a bet, so the first player to initiate action is making the first bet, a raise is the second bet and only a re-raise is the 3-bet. Post-flop 3-betting tends to have different implications to a pre-flop 3-bet. Used pre-flop, the strategy is often aimed at trying to assume control with a bluff, while a post-flop 3-bet is more indicative of strength. Generally, a player is representing a stronger hand the later the street on which they 3-bet.