I assume this is a passion project because I can't imagine getting funds for a project like this, specially when it might conflict with upstream licenses.
In any case congratulations to the developers.
You only needed one copy of the game and one PC for you and your friends to play together. Every class in school had at least one guy who had his own PC, who often invited friends over to play after class and since commie blocks are particularly walkable, we could all return home on our own.
I spent a good chunk of my preteen years like that.
[0] really badly translated to what directly translates back to "flaming buttcheeks".
H4 didn't make such a splash because even though it was better balanced, it felt like a downgrade in many respects.
I went to a different middle school so when I regained contact with my grade school friends they were already either exploring alcohol and sex or got sucked into WoW - in one case even both.
Anyway H3 left a mark on my generation thanks to its great replayability and social aspect that fit well into post-communist reality, with walkable, but hideous architecture and absent parents, majority of whom were working overtime(by western standards at least).
But H3 flaming buttocks was the best, because then you could talk and party whilst only one person at a time was distracted.
Ah, youth
Of course to play it you need to buy proprietary assets.
* If everything will look completely different from original game majority of fans not gonna like it even if quality gonna be really high. As you can guess fans of old games are really opinionated and hard to pleasure.
* If they looks too similar to original game then Ubisoft can righfully claim copyright infrigement so it will be impossible to distribute assets in Debian, Google Play and other stores.
Actually same problem is valid for any open source game reimplementations except simulators where you can claim that you copying some original and not someone else commercial game.
It's very much possible to just take VCMI and HoMM3 mechanics and create completely different game on top of it. Engine was intentionally designed for modding after all, but again it's unlikely that many fans will play it.