There isn't a Force Unleashed 3.
Its Darth Vader
Two... light side.... and semi dark side.
Jump down the staircase when you are at Vader and you will fight him witch will unlock the dark side ending.
You have to attack Darth Vader again instead of the Emperor in he final battle, this causes you to go to the Dark side.
If you mean "choose" the ending, yes you can. All you need to do is choose to fight Darth Vader(dark side) or The Emperor(light/bright side) after the first Darth Vader fight. All you need to do is walk/run to the enemy you want to fight.
Found out by myself jump down and fight Vader in stead of fighting the emperor.
If you're thinking about the Sith Assassin armor, you get that by helping the Emperor by going fully over to the Dark Side and killing Lord Vader as revenge for his betrayal :)
Once you have the mod installed you then have to finish the games last level as dark side by going after Darth Vader.
yes starkiller is a clone.you can read the full details on the wookieepedia; Dark Apprentice(clone)
At the end of the game, you must face Emperor Palpatine instead of fighting Darth Vader. This is the only way to complete the game light side and the only chance to really choose the light or dark side.
You will have to face Darth Vader at the end, instead of going for palpatine ( whilst he is choking kota ) jump down where you threw Darth Vader to continue the fight with him, but remember this is to the death there is no other enemies for you to kill to top up your health. Defeat him and you will get the alternative Sith ending along with ( if you haven't got already ) 20 GS if your on Xbox 360.
He choose the Force instead of The Dark Side.