There is no such thing as negative HP.
Example: 4 HP arm, lets say 14 total HP. 9 damage -> Arm drops to Zero and Total HP 14-8=6 - arm is useless and this incapacitates our hero. Next round, same arm takes 3 damage - arm still at Zero but Total HP drops to 3. last round same arm takes 5 damage - arms still Zero but Total HP drops to 0, PC is dead.
No lets assume that the third and final wound didn't happen. Our PC has a useless arm with two wounds, one wound at 4 damage and one at 3 damage. These could be healed with First Aid (one attempt per wound) or Magic. A triple damage score maims a limb which can only be healed with a 6 point Heal,