Ok, so I've decided to come back to Heroes 3 once again. I'm using HD Mod and I want to use GOG Galaxy to launch the game. From what I've read it's possible but needs a little tinkering. I tried editing goggame-1207658787.info file. This is what I had before editing:

"name": "Heroes of Might and Magic 3 Complete",
"path": "Heroes3.exe",
"type": "FileTask"

And this is what I have after editing:

"name": "Heroes of Might and Magic 3 Complete",
"path": "HD_Launcher.exe",
"type": "FileTask"

I've read that this should do it and the game should open the HD Mod launcher after clicking "Play" in GOG Galaxy. Unfortunately clicking "Play" does nothing. Auto updates are of course off. I'll be grateful for any advice how to make the HD Mod working with GOG Galaxy. Is there something else that has to be done? Any ideas? Clues? :)

Yep, that did the trick. :) I wasn't aware that you could add custom .exe files to games that are in your digital library and make it default. Thanks a lot! :)