All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

The map editor is launched using the executable file h3maped.exe, and the campaign editor is launched using h3ccmped.exe (both are located in the game folder).
For random maps generated directly from the game, you won't be able to change the heroes' skills. You can only do so if you create a random map in the editor and then edit the heroes' skills.
Also, during gameplay, your heroes can acquire new, useful skills. You can evaluate them and make another hero the main one, or even change the main hero multiple times throughout the game.