KozzyRock
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Considering my experience with similar issues, it's not certain that KozzyRock is playing the exact campaign you're thinking of, as he didn't specify the game version or the campaign name. It's possible that he's playing a WoG or HotA campaign, or some other fan-made mod that he found and downloaded. :)
The Restoration of Erathia
The first campaign is Long Live the Queen.
As I understand it, this is basically the very first thing that came out in Heroes 3 (not HD, of course). That's why I wrote
The third map of the very first campaign.
If we're talking about the topic, it seems that it wasn't a fountain, but a castle or a building in the castle that doubles magic when a hero visits the castle.
Added 1 minute ago
The third mission of the first campaign – is that the one where we get the Vampire Cloak? Does it have a magic spring?