'Defending against a 3bet' is the name poker players give to the situation where a player calls a 3bet with a marginal hand. So why should we ever defend against 3bets? Shouldn't we just fold all our marginal hands?
If we simply fold all our marginal hands when our opponents 3bet, we get exploited by our opponents 3bet bluffs. Over time folding all marginal holdings to a 3bet is a costly strategy, also it makes it very easy for our opponent to put us on a 'range of hands' when we do finally decide to play a 3bet pot. In poker we never want our opponents to be able to accurately put as on a 'range', because it allows them to play optimally and disadvantage us!
So when is a good time to 'defend' against a 3bet? The answer would be when we think our opponent is 3bet bluffing with a weak hand. This is because if our opponent has a weak hand pre-flop we can win the hand post-flop a good percentage of the time by:
1) Flopping the best hand.
2) Flopping a draw and win by semi-bluffing.
3) By bluffing our opponent off his weak hand.
It must be noted that our opponent who is 3bet bluffing is likely to be cbet bluffing the flop. If we think this is the case we should be re-raising our opponents c-bets on the flop with a high frequency! It is also very important to be in position when defending aganist 3bets, as we will need positional advantage to make our 'defend' profitable!
So when is it likely that our opponent is 3bet bluffing us? In poker today a lot of players do not give much respect to being raised from the button because they suspect that we are trying to steal the blinds a lot from this position. Therefore if someone 3bets a button open raise, we can suspect that he is 3bet bluffing more than would be usual. This would be an ideal situation to defend our inital raise against a 3bet!
We should not however just defend any hand when facing a 3bet though. Good hands to defend with are suited connectors, suited one gappers, small pocket pairs and suited hands with one high card. This is because these hands have a good degree of playability post-flop, in the sense that they connect with a good number of flops, giving us multiple ways of winning the pot post flop as outlined above.