The standard 3-bet size is usually between 3x and 4x, so if he opens to 3bb the 3bet would be around 9bb-12bb. There are times to use slightly different sizing (against fish or odd-stacked players for example), but 3betting between 3x-4x will serve you well as a default.

For example, suppose another player raises preflop, leaving you with the option to call, fold, or re-raise. If you choose to re-raise, that action is known as a 3-bet, and you are the 3-bettor in the hand.