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

I can't install the HD mod on Heroes 7. I tried versions 2.95 and 2.98 - in any case, the game crashes when I launch it. What should I do?

It's unlikely to be a problem with the mod. Heroes wasn't designed to run on Windows 7. (It wasn't even designed for Windows XP...) You should try running it through a Windows XP virtual machine.

P.S. By the way, a question for baratorch: is there a way to use a mod to enable Heroes to run on Windows 7? Heroes will still be popular for a long time, but Windows XP is essentially dead...