I am looking for maps for Heroes III.

scoute, it seems that here is the map you were looking for – everything matches your description.

Eight kingdoms fighting against each other for control over Treasure Island and ultimate domination. End this Great War by crushing your opponents. And don't forget about the treasure in the Middle.

I found it in the collection "1500+ maps for Heroes of Might & Magic 3" (I downloaded it, although I don't like such collections: they contain a chaotic mix, many duplicates, and no additional information). There are also collections "3500+ maps for Heroes of Might & Magic 3" and even, supposedly, "16000 maps for Heroes of Might & Magic 3" (I haven't seen one myself, but one person wrote that he has it). However, it's not just about greed: sometimes you can find good maps in these collections that are not available in online collections.