Try HotA: <span class="podkreslenie">click here</span>.

About resolution mod try Heroes 3 HD: <span class="podkreslenie">click here</span>.

P.S. HotA is the best known mod for HoMM 3 and very delicate in terms of gameplay/bug fixes and added content.

Thanks :-)
HD works great. HotA also seems great - true to the original.
But when I load HotA there is only the new Campaign. Is it possible to play the original 3 campaigns with bug fixes/tweaks from HotA?

I have GoG HoMM3 Complete

There is also The Succession Wars Mod and the VCMI source port. You can watch all of them (for H3 and also other heroes) in this topic: http://www.gog.com/forum/heroes_of_might_and_magic_series/homm_series_mods

The Succession Wars Mod is also WoG-based, but the current team tends to develop the classical aspects much more. I don't even know what will happen to WoG legacy in future, since WoG can be also used just as a platform for modding.

Heroes of Might and Magic 3 is not a very mod-friendly game. The work done by the WOG-team have made many things possible, but as the HOA-team found out with their struggles, it still is very difficult work.

First of, Horn of the Abyss is more than a mod - it's an unofficial add-on.

Here is an important mod you might have missed:

https://www.youtube.com/watch?v=BaTjQ3rDKXU

Use it with Heroes 3 HD mod and after playing it for a while, you might want to never "look back" at the original sizes of the creatures. Unless you think that imps and goblins should be as big as giants or cyclops like in the original...

Slightly OT but has anybody tried this balance mod for HoMM II (2)?
It seems fairly well thought out but having only played through the campaign I'm not exactly a pro at this one.
http://www.celestialheavens.com/forum/7/8724

I've installed it with the windows GOG version (just replace the .exe) and it seems to be working fine despite that note in the forum thread.