I can neither agree or disagree with your assessment due to the many existing variables, but I would offer the supposition that it might hinge on the location of the shooting. If it were in anti-gun areas like New York or Chicago, you may be correct. If in Texas or Georgia (and states with the Castle Doctrine in place), maybe not so much. Especially if there is a large disparity of force between the assailant and the intended victim.
If you look from the standpoint of "shooting until the threat stops" and the third shot was the lethal shot (as in your statement), then it could be argued by a good lawyer that the threat still existed even after the second shot. I know of a cop who was shot by a suspect who had already been shot and was on the ground via a second (unknown) weapon on his person.
Two things to remember in a shooting situation, once the decision is made to react to a threat by shooting:
1- Just because you don't see a weapon doesn't mean there isn't one, and
2- Since the decision to shoot in the first place was already in motion, and will likely be the motivating factor in questioning the legality of the shooting, then shoot until you no longer feel the person is a threat.
This is my opinion only, and not that of the show or producers or the OC.
As in all things, your mileage may vary.