Here is where i learned basics:
http://www.heroesportal.net/library/index.php once you get to that site DO NOT translate it, click on the БИБЛИОТЕКА(library), after that under "Герои Меча и Магии III" you will find guides for mechanics, magic etc. once you click for example "Физика мира" only than you should translate page to English(using browser translate not site translate).

At first it might look boring as most of the stuff you probably know, but you will notice that there are some minor details that separate top players from mediocre, and those details can often win you games(for example you might have lost step with your hero in the beginning because you carelessly picked up resources, who knows maybe that 1 step will be needed to reach opponent's main hero without army in the late game).