It has to be a 1:1 clone really?
The single most important question to ask here.
I believe the answer is yes, unfortunately.
You need to realise how much of a steaming pile of utter shit the HoMM3 hardcore fandom truly is. These people are soulless zombies chasing something they'll never get back in their life. They roam the realm looking for their lost paradise, shunning literally anybody who doesn't oblige to their lunacy.
With this in mind, yes, the game needs to be a carbon copy of HoMM3. Because, you see, their faith is based almost entirely on the realisation that HoMM3 is unsurpassed and any attempt at changing the formula is, to put it simply, an heresy punishable by (social) death. They are the true last boss of gatekeeping, the Dark Souls of retarded fanboyism, the ultimate step on the staircase to insanity.
Their modus operandi is always the same : does someone try to scratch the HoMM3 legacy ? If no, go to sleep. If yes, harass them. Harass them until they understand that their project isn't a carbon copy of HoMM3 and therefore flawed. If it happens to be a carbon copy, just ignore it because HoMM3 already exists and there's literally 0 reason to jump ship.
I really, for the life of me, will never understand these spastic fellows. They don't seem to understand the utter stupidity of their gatekeeping even. But utlimately, if you don't make a carbon copy, you'll get shit on.
Now picture this : you work on a game that only a handful of slavic peasants will pay attention to, and the only form of support they can give you is if you actually just remake HoMM3 pretty much exactly as it was, because the slightest difference will be condemned by their wise council of autists. Why would you not indulge in their madness, actually make a carbon copy, cash in and fuck off ?