Win10 MSI GT72 laptop

It just shows a small black window (1/4 of the whole screen) after booted up. Clicking on the small window, "Heros 3 is not responding" shows up.

Anybody knows how to solve this problem?

Thanks.

Try using a compatibility mode. I use Win 98/Win ME compatibility mode for HOMMIV. You can try with this or go with Win 95. It worked with HOMMIV, but I have never tried with HOMMIII (I have the HD Edition of HOMMIII).

How to use a compatibility mode: go to the HOMMIII folder (it`s at GOG Galaxy/Games/HOMM 3) and right-click at the file Heroes3.exe (I guess that`s the name). Choose "Properties", Then go to the tab "Compatibility" and choose "Windows 98".

I don't think you got a decent answer yet so I'm gonna add this instead:
Try using the great GL Wrapper made by Verok for the first 4 Heroes games.It works great on windows 10 for me.
Here's the English translated link (he is Russian):
https://translate.google.com/translate?hl=en&sl=ru&tl=en&u=https%3A%2F%2Fwww.old-games.ru%2Fforum%2Fthreads%2Fserija-igr-heroes-of-might-and-magic-gl-wrapper-patches.80503%2F

Just download the file for Heroes III and put it where your Heroes 3 exe is.

I hope this fixes your problem!