My 3-year-old son likes Star Wars, too. He walks around with a paint stirrer, claiming it's his "red light saber" (Darth Patrick!), and has to sleep with his brothers' action figures.
I wouldn't take a young child to see Episode 3, or any of them, in a theater, but when they come out on DVD it might be okay. It's mostly just flashing lights to them. Young children really aren't paying much attention, and they don't understand the plot, or even much of the dialogue. At home, if there's a visual they might find disturbing, they can leave the room.
I think of Star Wars-type movie violence as being sort of like nature documentaries, to a child ... I remember when my oldest daughter (now 14) was about 2, she was watching a nature show where cheetahs were disembowelling an antelope - loud chomping, guts all over the landscape - and she said, "Oh, look, the kitties are eating their dinner! Nice kitties!"
My son was two when the first one came out. We thought maybe he could sleep through it. He did -- with me holding him in the lobby. It was weeks before I got another chance to see it.