Maybe it's a bit late to make this guide but is it really? Honestly, it's better late than never.
How to Get the Missing Content on Steam
This guide is pretty simple, even if you're not a computer tech, there is no way you'll screw up everything, if you do, well, you didn't follow the guide, you can always verify the cache of the game if you screwed up but really, you shouldn't.
What you'll need is the original version that contains everything, either CD, the GoG edition or if you got it from somewhere else.
Just be sure that it has the content to begin with, otherwise it won't really be worth it.
I've tried multiple things to come at this point, but in the end, only something this simple was required, i tried using multiple parameters, playing around and i've end up thinking, maybe something this simple would actually work? And it sure do.
It still is sad that Ubisoft are too incompetent and greedy to do anything right but anyway, if you are interested in playing it on Steam so you can count the hours you've played while having the complete experience, this is the only way i could find so far.
What You'll Get by Doing This
You'll get 7x (random number since i'm actually very bad with numbers) content as the Steam edition.
So here is some of what you'll get:
- Restoration of Erathia, Armagedons blade, and Shadow of Death, Armagedon and Shadow of Death campaigns.
- Greater map pool.
- Random Generated/Randomized maps.
- More campaign options.
- More items.
- Conflux Town.
- More creatures.
- HD mod.
- Lot, lot, and lot of mods.
- Other stuff.
Now, what you need to do is simple, since i own the GoG version, i'll do it with the GoG version, the files from the place you got it may be different but it should be similar.
Take everything that's in the folder of the game that has everything, for example, mine is located here: \GalaxyClient\Games\HoMM 3 Complete, if you got the GoG version installed using the Galaxy Client, it should be at the same place.
You'll place all of the files inside that folder where your Heroes of Might & Magic III HD/Steam edition is at.
Should look like this: SteamLibrary\steamapps\common\Heroes of Might & Magic III - HD Edition.
Once that is done, rename HOMM3 2.0.exe to HOMM3 2.0.exe.bak or whatever you want, you can even remove it if you want but i'd keep a copy in case you want to return to the HD edition which i honestly wouldn't so i have personally removed it but it's up to you.
Copy the name "HOMM3 2.0", make a copy of Heroes3.exe which is the executable name for the GoG edition.
Rename that copy to HOMM3 2.0 and that's it.
Do not touch the launcher or else it won't work.
Important Notice: The only problem i encounter is that the Steam overlay doesn't work, i'll be looking into it at some date later, i haven't tried the HD mod yet but the factions mods seems to be working, maybe the HD mod will make the Steam overlay mod, i doubt it actually but who knows.