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Have you encountered this problem?
It's because your screen resolution is higher than the game's resolution,
so it only shows you the right side of the game!
There are two solutions:
1) Simply set the screen scaling to 100% - you can find this by right-clicking on the desktop, selecting "Personalization," then "Display." You'll see options like 100%, 150%, etc. at the top - choose 100%, and everything should be fine!
2) A simpler solution: right-click on the Heroes 3 shortcut (preferably not from the desktop, but from the folder), select the "Compatibility" tab, and at the bottom, check the box "Disable display scaling on high DPI settings." This should also fix the issue. You can also enable compatibility with Windows XP at the same time! =)