Heroes of Might and Magic III on iOS and Android.

You are mistaken; it is impossible to create a complete clone with the original graphics without violating copyright; if it were possible, they would have done it.

Okay, let's assume that. But then, how do the following projects exist:
WOG
HotA
heroesland
Heroes 3 on Android?
You can find more information in the online heroes section, in the ports section, and so on. By the way, this topic would also be better suited for the ports section, in my humble opinion.