In general, you should just call a bet when you have a decent hand, but one that isn't quite strong enough to raise with unless you're turning your hand into a bluff. In some cases, you might have a decent hand such as middle pair but are concerned your opponent has you beat.

Don't call 3-bets against short-stacks. Suited connectors can profitably call 3-bets with deep stacks pretty often, but they lose a lot of their value when the effective stack size drops to around 50 blinds. This happens because their once-great implied odds are diminished.