I have never personally dealt with editing Super Mario 3, but I have dealt with your level of frustration, and still do to this day.
ROMhacking, I have discovered, is a never-ending learning process. I've been plugging away at it in one form or another for the past six-and-a-half years and even now I still feel like a beginner at it in many respects. Despite all this, let's see if we can help you out a bit and get you pointed in the right direction...
First, you're going to need to get yourself some useful tools, as editing things in FCEU, as useful as it can be, can only take you so far. I would recommend a a good hex editor so that you can deal with the ROM directly. I have three I keep lying around which I use for various purposes...Hexecute
, which I keep around for easy-as-pie copying and pasting, XV132
which also has some useful editing functions my other ones don't have, and HexCmp
which has proven extremely useful to me as it can load two ROMS at once and highlight the differences between the two. You'll probably have to fork out a little dough for HexCmp, but the others, as far as I can tell, are free.
Another good tool you can use for finding data in a ROM is a file corrupter. I have personally found Corrupster
to be the most easy and straightforward to use. It will basically let you corrupt a byte in a ROM, or a range of bytes, with either random values or a specific range of values so that you can play the ROM and see what changed, and if what got fudged up is what you're looking for, you're on the right track. It will be up to you from that point on to figure out the format in that range of bytes in order to manipulate the ROM to do what you want. A ROM corrupter may not tell you how to change what you want to change but it's definitely useful in telling you where
to look. As far as Super Mario Bros. 3 goes, I'm not sure if you are trying to change the actual music data itself or change what music gets played in what stage, (in which case you will have to deal with pointers,) but Corrupster should get you well on your way.
I sincerely hope that helps, given my limited experience with what you're trying to do. Good luck to you.
Cheers and thumbs up.