A 3 bet poker situation is when there’s a re-raise over the original raise. In other words, this is when there’s a bet, raise, and a re-raise again.
So, the re-raise is technically the 3 bet. In this context, if there’s another re-raise after this bet, it’s called a 4 bet, and so on as long as players are raising back and forth.
For example, let’s say you’re playing a cash game and you’re in the button poker position. After a player before you raises, you see that you have a good hand and decide to re-raise, effectively making the 3-bet.