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

On Heroes TV, there are two links:
Download Heroes 3 SOD + pre-installed HD mod (852 MB)
Heroes 3 SOD + HD mod by JustObserver (899 MB)
I downloaded it from this link - Heroes HD mod page https://sites.google.com/site/heroes3hd/rus/download
I assume it's all the same thing, or does Observer have some changes?