What is a 3-bet?
A 3-bet is the first re-raise before the flop. The big blind is bet one, the open raise is bet two — so raising the raiser makes it three.
Why 3-bet at all?
Three reasons: value — big hands want a big pot against worse hands that call; isolation — you'd rather play aces against one caller than four; and initiative — the 3-bettor's story of strength wins many flops with one continuation bet.
A starter range that won't hurt you
Versus an early-position open: QQ+ and AK — that's it. Versus a cutoff or button open (their range is wide): add JJ, TT and AQ. Everything else: call or fold. 3-bet bluffing is real and profitable — after the value core is automatic, not before.
Sizing
Simple rule: 3× the open in position, 4× out of position. They open to 3bb — make it ~9bb when you act behind them, ~12bb from the blinds. Min-3-bets invite calls from everything and defeat the purpose.
The expensive mistakes
Flat-calling with AA "to trap" (you trap yourself into a 5-way flop); min-raising; 3-betting medium pairs into early-position opens (their range is exactly the hands that beat you); and folding to every 4-bet after building the pot — if you 3-bet QQ+/AK, you usually continue.
Ready to practice? Run the 3-bet-or-fold trainer — and make sure your opening ranges are automatic first.