[BanditAssasin]

It's putting in a 3rd bet / raise...

So preflop, if someone raises and you then re-raise, that's a 3bet (since the initial blind bet to be called counts as the 1st bet, the initial raise is the 2nd bet, your reraise is the 3rd bet).

Postflop it's if someone bets (1bet), someone raises (2bets), and you raise more it's 3 bets - so you have 3bet.