I have investigated a little bit further regarding Galthran and other heroes. The switch between enable or disable a hero by "default" must be somewhere in homm3/hota map editor or homm3/hota exe file. I have excluded all other data files and its neither in the hero starting sskills, starting spells etc... part of the executable files (linked above - when you look the files with hex editor).
Is it possible when you start a new game and choose towns/heroes/resource the game.exe file somehow use map editor.exe to set the chosen town/hero? Because in this case I think the "default hero switch" must be somewhere in the map editor.exe file?
Any ideas, suggestions on this one?