HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

In the second scenario, observing another player (an opponent) battling against AI/neutrals is akin to cheating, as it provides accurate information about the opponent: their skills, troops, spells, and artifacts (the last two points are not always applicable, but possible when they are used).
The same applies to the first scenario. An exception: perhaps it would be possible to organize a viewing of a battle of an ally.