GLeBaTi
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1) It depends on what hostile troops you will be fighting and whether you have expert magic. For example, if a dragon attacks archers, one stack of archers cannot shoot, while another can.
2) There is no difference.
3) HOTA and WOG are mods; WOG appeared earlier, but HOTA is closer to the original (fewer changes): mainly, it's a new city (The Docks) and everything related to it. And also, in HOTA, the grass is greener (this is my favorite joke). :)
More details:
https://ru.wikipedia.org/wiki/Heroes_of_Might_and_Magic_III:_In_the_Wake_of_Gods
https://ru.wikipedia.org/wiki/Heroes_of_Might_and_Magic_III:_Horn_of_the_Abyss
I can only advise one thing: FIRST learn to play confidently in Complete Collection or The Shadow of Death, and only then, if you want, move on to HOTA or WOG.