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 >>/81402/
Could be the case.
The HD addon for Heroes III doesn't do that. It scales the adventure map, by effectively moving the camera back, increasing your field of view.
Which also means the battlefield is still 4x3, leaving a shadowy background around the edges. No changing the aspect ratio there.
Everything else is upscaled using an algorithm of your choice. Just go with whatever they recommend.
This is how the HD addon looks like