1. Most HIV patients are young. Thus youth causes HIV. ANSWER APPEAL TO IGNORANCE

Question

2. There is no extraterrestrial life. After all, no one has ever found observable data to support the claim that such life exists.
ANSWER: APPEAL TO IGNORANCE

3. The attorneys for the prosecution were not able to establish beyond a reasonable doubt that Hinckley was sane when he fired at President Reagan. We know this because Hinckley's own lawyers declared that at the end of the trial. So we have no choice but to conclude that he was insane.
ANSWER: HASTY GENERALIZATION

Answer 1

I agree with #2 - ignorance
#1 - ignorance or hasty conclusion? Probably both.
#3 - When both sides agree, is it hasty generalization? I don't think so.

Answer 2

So #3 would be a appeal to unqualified authority.