„For“ sounds better to my ear in both sentences and I can’t tell any difference between „last“ and „past“ in either sentence. I’m sure many people would say „in the last three years“ and no one would hear the difference. I just think I‘d say „for“ myself. It might be that it’s just used more. I don’t know of a grammatical rule why „in“ would be wrong, however. Again, I’m just drawing on what I’ve heard more often, just because it sounds better to me. It’s one of those „I can’t tell you why, it’s just that one sounds better“ answers. Sorry.

"in" is basically a short form of "within". They both work. All sentences are correct.

To me there is a bit of nuance between "in the last three years" and "for the last three years". When you say "for", it sounds like they have been working hard the whole time, while "in" could imply that it hasn't been the entire time. There is no difference between saying "last" and "past", in my opinion.