0) Holy moly avoid changing the default text color/font when posting on forums.
1) Yes, it's possible. In general if the question is "is this possible" the answer is always "yes" unless you are trying to do something that exceeds the capabilities of the system you're working on.
2) That's not typically how it's done. NSFs are basically mini-programs that execute code, use RAM, etc. Trying to insert an NSF into a game is like trying to take a chunk of code from one game and cram it into another. More often than not that's going to lead to all sorts of conflicts and problems.
Usually the way this is done is you figure out how the game stores it's music data, then edit it in place. There have been TONS of MM3 hacks that change the music, so I'm sure the music format is already documented somewhere. I'm too lazy to check for myself, but there may be a doc on this site.
3) You can use VRC6 but that is a fairly huge undertaking which you're probably not ready for. It would involve rewriting chunks of the game code to use the new mapper, as well as pretty much rewriting an entirely new music engine from scratch to use the new audio channels.
PS) I think Megaman 3+ all use the same music engine. So if you find a document explaining it, you should be able to edit any of them. MM1 and MM2 use a different engine though. But this is just from my recollection, I could be wrong.