Try playing the HD add-on with the HotA (horn of the abyss) espansion! There are 2 new factions now which are well balanced.
And, they look beautiful.
Its wonderful what a dedicated community can achieve. Kudos to all of them!
TIP: if you get the game on GOG. It will run just fine in any modern Windows setup (even MAC OS i believe)
Another vote for the Horn of the Abyss! It is a community pack that keeps the HOMM3 gameplay spirit but adds a lot of minor UI enhancements fixing various pain points.
And if you like insanely complex scenarios, check out HotA user maps on maps4heroes.