HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Hmm, but how do you attach it to the GR now? Previously, the installer automatically created an HD executable for each version of Heroes3.exe, and you could simply rename the desired one and use it in GR. But now there's only one launcher, and you have to launch it with parameters... goodbye GR :)