I'm sure this is a very naive question, but is there any legal distinction between two in the chest and one in the leg? Realistically, I don't think one in the leg would be appropriate in that situation, but is use of a gun always considered Lethal Force in the eyes of the law?
Lethal force is lethal force. You can't shoot someone gently. The only time you are legally justified in using deadly force is if you, or some other innocent, is about to die. If they are about to die, what you are doing shooting them in the leg?
Shoot center of mass (upper chest). If that doesn't stop them, consider the head. Legs are actually pretty hard to hit (they are small and they move a lot) and shooting someone in the leg probably won't stop them.
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Have there been any cases where someone (a civilian) was involved in a shooting here and didn't wind up in court over it? I honestly don't know.
Yes. That doesn't mean that they didn't end up spending a bunch of money on a lawyer, however. Lots of legal work can end up being required to convince the police department and/or DA not to press charges.
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Is that from a statute or case law?
Case law. Read: http://www.amazon.com/The-Law-Self-Defense-Indispensable/dp/0988867702