Is Heroes 3 a sport, like chess? How does the planning differ?

You are under an illusion. I asked what the current rules are in chess, combined with how they can be broken.
"describe an example of how to perform an action in chess that is forbidden to both players."
You didn't say a word about the rules ("an action forbidden to both players").

I don't know, maybe we're speaking different languages, or one of us is being really dense. It's obvious that using hints (including from a computer) is a forbidden action that will give one of the players an advantage (or victory). Apart from obtaining information from outside sources, there are no other ways to unfairly win in chess.

Naturally, in Heroes, there are more opportunities and methods to break the rules, but the degree to which they affect the outcome is less.