The model has to make 3 defense rolls. Each of them that fails is a wound. If that model only has 1 wound, then obviously it is destroyed.
This is different than if it were a Deadly weapon. If it were Deadly(3), that model only makes ONE defense roll. If it passes, no damage. If it fails, then 3 damage (with 2 damage going to waste, assuming the model has 1 wound).