These are all the official campaigns released by 3DO, which include Restoration of Erathia, Armageddon's Blade, Shadow of Death and Heroes Chronicles.
To install, extract the included Maps folder over your existing Maps folder in your main Heroes 3 folder (it shouldn't overwrite anything). To play, start the game, press New Game, press Custom Campaigns, select desired campaign to start.
Why they simply don't show up in my GOG version of the game?
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