Here are some articles regarding Heroes 3 that may prove to be useful.

Gamedev on damage calculation

Gus Smedstad’s description of the Heroes 3 random map generator

I’m still looking for some good AI articles regarding heroes. If anyone else has some good reading materials, feel free to post them!

Interesting, especially description of RMG. Thanks
Plenty of information about H3 (including descriptions of all objects and units) can be found at HMM3 wiki.