After playing Myth III through fairly good, I discovered
that old cheat codes still work in this game. So, if
you want to get access to all maps then holding Shift
button down, click on "New Game". If you want to win
the mission then you can hold down simultaneously three
buttons - Ctrl+Alt "+" on the NumPad, but if you want to
lose the mission then you should do all the same but
with "-" of the NumPad.