This is a feature-rich Rockman 3 editor that also works with Mega Man III. It supports editing screens and the structures, blocks, and tiles that make them up, along with palette and enemy editors. One unique feature is that it allows you to edit two screens at the same time. This program is in Japanese but it is very easy to use even without being able to read the text.
File download:
Most definitely not beginner friendly. The UI is broken, a stage is impossible to edit, the previewed visuals are innacurate, swapping sprites and enemies is a chore, you get the drill. You’d have no reason to use this if it wasn’t the only editor for this game out there.
I uploaded a few of these MM editors from the same Japanese guy, but you might be able to find a few older tools in one of the RHDN archives. I know that GitHub has a tool called CadEditor (“universal level editor”) which has limited support for this game.
https://github.com/spiiin/CadEditor
If you find a workaround for something, you should be able to make a main page for anything old or free to distribute. Sometimes it’s a case of using a very hacky approach to modding when an all-in-one solution isn’t around.