Just downlaod a map of Heroes 3 from maps4heroes website, extracted and pasted in this folder:

C:\Program Files (x86)\GOG Galaxy\Games\HoMM 3 Complete\Maps

In the game, the map does not appear in the list. I've tried differents maps, same error. The game is the classic 3 (not HD!)

What should I do to resolve it?

The strange thing is that I tried removing one of the game's default maps from the Maps folder .. and when I restarted it it was actually gone! Then I put it back in and it reappeared.

Try running as administrator.

It doesn't work

It may be storing maps and saves in alternate directories. It will be in the user tree but don't remember where.

Maybe. User/appdata/...

Look for virtual store subdirectory

Or Uninstaller gMe and reinstall in a non protected location. (Not in program files)

Sorry for typos. I'm on my phone and font is so small I can hardly read.

It's strange because if I right click on the game - open file path - it redirects me to the directory with all the folders (mods, mp3, data etc..)

The strange thing is that I tried removing one of the game's default maps from the Maps folder .. and when I restarted it it was actually gone! Then I put it back in and it reappeared.

Does the map have an .h3m extension?

yes

1) Check that all maps in the maps folder are offered in the game. Maybe you're looking at the wrong folder. If you have or had multiple installations of H3, there might be other locations of maps. Search your computer for h3m files.
2) If your previous post about removing/restoring a map caused it to dis-/re- appear in the game, then you should be in the right folder.
3) It's possible the map was made for hota or some other mod and so won't work for classic H3. Check the source you got the map from to see if there's anything indicating that.
4) I know it's spoilery, but can you open it in the map editor? If you can't, that further suggests it's not suitable for classic.

I tried, you are right! The map editor doesn't open them. It tells me they have a different version ... but then I should update the map editor? How do I do that?

Look the image attached

Okey on maps4heroes website all the maps are avaible for the extension "Horn of the Abyss" (I don't know why) but in the complete edition there isn't it.

I will downlaod that extension and try again.

I don't know what you mean "in the complete edition there isn't it." What isn't what?

If you have downloaded a horn of the abyss (hota) map, you will not be able to play it without the hota mod. H3 classic/ vanilla/ complete will never be able to open it.

People have different opinions about WOG, ERA, HD and HOTA mods... some like them, some don't. If you want to play a more polished version of H3, with several useful keyboard commands, but is nearly identical to vanilla H3, then you may enjoy hota. If nothing else, it will let you play all those new hota maps!

EDIT

The reason I asked about the map editor, is because you need the hota map editor to open hota maps. Since your map wouldn't open in the game OR editor, it seemed likely to be a hota map. Don't bother downloading the hota map editor. It won't help if you're not playing hota.

Either download hota and play those maps, or find new maps that are not hota. There are tons of maps out there.