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.)
A 3-bet as most players use the term means the act of putting in the third bet, technically the second raise , the 3-bet during any given round of action. It's only in recent years that the term has become popular, indicative of its use during online play.