A raise against an open-raise.
The blind is the first bet (1-bet) and the open is the 2-bet, so the next raise is the 3-bet. As a guide, size it 3 times the open in position and 4 times out of position. Example. CO opens to 2.5bb and the BTN 3-bets to 8bb. Live players who don't fold open only strong hands, so you don't need bluff 3-bets: 3-bet JJ+ and AK for value (TT is borderline; call with AJs, KQs, and AQo).