Is there an editor for Might and Magic III that works with Windows 10?

I recommend the good old UGE (universal game editor), which works fine in dosbox.

I might upload my custom made UGE modules on some website soon. GOG won't let me attach any file other than images.
I made my own modules for MM1-5, and also X-COM ufo defense.

In the meantime, I recommend any old hex editor.
If you need to know a specific offset, ask away, chances are I have it.