So, I played through the first campaign ("Long live the queen"), then started a second one ("Dungeons and Devils") picking a "New Game" option, instead of (how I was maybe supposed to, I don't know, if I was, I think the game should have been more straightforward about it) loeading the save the game made at the end of the first one.

Now, after loading the first campaign's save, it shows only "Long live the queen" as completed, and loading the second one's shows only "Dungeons and Devils" as completed.

Does that mean, that if I now complete the third one ("Spoils of War"), I won't have the other Restoration of Erathia's campaigns unlocked, and I'll have to pick one of the saves, and complete the other campaign starting from it?
(I can do that, though I still think the game should have been more clear about letting the player know if that's the case)

Yes!

Unfortunately, you have to replay one of the campaigns. Do it now before it's too late.

The game should have been more clear on this, but you don't select "New Game" after finishing a campaign, instead you continue by clicking on the campaign straight away.

All right, thanks. :)

This gets me every single time. I always go through all four campaigns without realizing I'm doing them only individually, that they aren't combining. Then after the fourth I stare at it wondering how to go on because the instructions say they are supposed to be interconnected.

Then I realize the game isn't set up the way I understood, that completing a episode doesn't unlock it, doesn't complete it. This isn't StarFox 64. And then I remember having this happen to me a few years before. And a few years before that. And a few years before that.

This is partly why I have never made it through the whole game of HoMM3. The other part is that I'm always totally upset I didn't understand the interface even though I did understand what was suppose to happen, then I rant about poor user interface designs on computers in general, then emotionally collapse into indecision and usually lack of time to do over any of them at that moment. Then a few years go by and I recall how much I enjoyed the individual games and somehow totally not recalling what happened when I tried the campaign, probably because I don't want to remember experiencing that part. Inevitably I get stumped by again. *sigh* Maybe one of these decades…

I wonder if there's a tool for combining separate saves with single completed campaigns into one, with all of them completed. If there isn't - it should be, and I suppose there are people with expertise needed for that, considering how much has been done with HoMM3 modding already.