A source of aggravation for me when I played HoMM3 back in the day was that for most of the campaign, the difficulty setting for each mission was hard-coded -- no tackling Long Live the Queen on Impossible to keep things spicy, nor any chance to just vibe your way through Armageddon's Blade on Easy.

It turns out that this isn't actually that hard to fix. Unfortunately, the forums won't let me post links, so I can't link you to pre-patched campaign files, but here's how to do it at home:
- Download MMArchive and unpack it somewhere
- Use it to extract all the .h3c files from Data/H3ab_bmp.lod
- Do the same with Data/H3bitmap.lod. Some of the files have the same name as files in H3ab_bmp; overwrite the H3ab files with the H3bitmap ones.¹
- You should now have 21 h3c files. Open each one using h3ccmped.exe (it's already in your HoMM3 directory), press ctrl-enter to bring up the campaign settings, and if "user selectable difficulty" isn't enabled, enable it and save your changes.
- Drop all the edited h3c files in the Maps/ directory.

The campaigns in Maps/ will override the ones in the packages in Data/ and you should now have selectable difficulty in every campaign. To undo the changes, just delete the h3c files from Maps/ and it'll go back to the ones the game came with.

You can also do this on Linux, either using Wine directly, or using Heroic Launcher (which I recommend for GOG regardless) and the "run EXE in prefix" option to run MMArchive and h3ccmped.exe.

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¹ For some reason, H3ab_bmp.lod and H3bitmap.lod both contain a complete copy of Restoration of Erathia; however, only the one in H3Bbitmap.lod is actually used by the game.