Walkthrough of HoMM3: SoD Campaigns

You take the desired map, open it in the map editor, and export the (text) file from the map. Beforehand, open the campaign in the campaign editor and export the maps from it. The editors mentioned above are located in the game's folder. I downloaded the Tanrum campaigns (8 of them) somewhere online – search for them. The remaining campaigns can be extracted from the game files, but you'll need a special utility for that.