A 3-bet is the third bet in a poker sequence, in which a player re-raises after the initial pre-flop raise, or 2-bet. (The blind payment is the first bet in Texas Hold'em and Omaha.)
The term has its origins in fixed-limit games where an initial raise is worth two bets, then the reraise is equal to three and so on. Similarly in no-limit games the big blind is the first (forced) bet, the first raise is the second, and the first reraise a three-bet.