Not sure exactly what you mean by three main scenes, but I'm guessing you mean everything but the "transmission" sequence. If so, you have 2 choices (Darth Vader, Stormtroopers), 3 choices (Tatooine, Hoth, Kashyyyk) and 3 choices (Death Star, Naboo, Coruscant). Among these there are 2*3*3 = 18...