AT as BB facing a raise

[Chris Daddy Cool]

I don't think 3-betting is correct in this spot because you are either way ahead or way behind.

If you are way ahead of say, QQ, and this player is the type to be able to fold, then 3-betting is clearly incorrect. Let him "value" bet his worser hand and incorrectly try to catch a Q. But if he will call you down, then I can see 3-betting.

If you are way behind, then it's pretty obvious why you shouldn't 3-bet. Either way it's a case of losing the least when behind and winning the maximum when ahead.

Either way though, you must see a showdown in this hand. You simply cannot fold this.