suppose a model has 3 wounds - the first time he suffers a wound your opponent doesn't make injury roll, and he is now reduced to 2 wounds. Next time he's wounded your opponent still doesn't make an injury roll, and he is then reduced to 1 wound. Only at the point that a model has only 1 wound remaining does his opponent get to make injury rolls against him, with a few exceptions which are usually stated pretty clearly.
Now, in the event that a model with 3 wounds suffers 3 wounds simultaneously (i.e. a melee opponent with three attacks strikes and wounds him successfully all 3 times), then the opponent will get 1 injury roll. If he had inflicted 4 wounds, he would get 2 injury rolls, etc. etc. Assuming he is not OoA after this, he has only 1 wound when he stands back up.
The only ways that I know of to restore wounds during a game are either Healing Herbs or one of the Hexes that the Witch hired sword can learn.