A 3-bet is the first re-raise before the flop. The open is the raise; your re-raise is the 3-bet. Too few and they open into you for free. Too many and their 4-bets print.
Value — you want a call from worse (QQ+, AK, sometimes JJ/AQ depending on the opener). Bluff — you want a fold, usually with hands that block their continue (A5s, KTs) and play well when called.
A solid 6-max baseline is about 7% 3-bets. Under ~4% you are a punching bag. Over ~11% you are lighting money on fire unless the table is folding too much (then you are exploiting — on purpose).
In position, ~3× their open is a common cash size. Out of position, bigger (~3.5–4×) so they cannot cheaply realize equity. Versus a steal (BTN/SB open), you can 3-bet wider; versus UTG, tighter.
When there is an open and a caller, a 3-bet is a squeeze — extra dead money, a capped flatting range. That is one of the most profitable 3-bets in the game.
Charts for 3-bet vs each opener live under Range charts. Train will grade the live decision — including when the right play is a call or a fold, not a hero 3-bet.
The first re-raise before the flop. The open is the raise; your re-raise is the 3-bet.
A solid 6-max baseline is about 7%. Much lower and opponents open freely; much higher and you get 4-bet light.