I have mixed feelings about when 3 betting is most useful. Typically there are 3 situations when you can 3 bet, and they are.

1) You have AA/KK and some one open raises in front of you.

If you are an agressive player you can include AK to 3 bet. The problem I have with this scenario is that this would of been a perfect time to stack someone, and by 3 betting you usually just win the blinds and the raise, unless of course he plays back at you/thinks you are a donkey/fish and has QQ/JJ or even AK/AQ. I have seen Donkeys/fish do this too.

2) The Squeeze play.

This is the most agressive play out there and can be very dangerous. Someone open raises, and has a caller. You've yet to act and when its your turn u 3 bet them both. Object here is how strong was the initial raiser, not worring about the caller (he usually is pretty week and guaranteed to fold)... They both fold...You dont need a premium hand to do this, and usually is player dependant. (Stats/HUD helps alot here)

3) Final and last play is my favorite and I use it alot. The Resteal.......

Again player dependent. I am in the blinds and am always aware of the the stealing % of the players to my right sitting in the cutoff and button. They tend to raise way to much and a 3 bet from the blind is extremely profitable taking down their original raise and the other blind. I like this one the most and use it the most.

Do you have anyother scenarios? Which do you use or not use?