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

That's true, the hero's mana isn't visible in battle.

In online play, this can be critical, and the cost of some spells isn't visible in the spellbook either. I specifically downloaded the complete version so I could play properly. You can download it, for example, here: http://www.ex.ua/view/70065, and HD textures can be installed on it; it's been tested :D.
However, it's 820 MB here; Rebellius threatened to upload his working build of 300+ MB.