Our DD went for the first time as a 3 yrs. old (in 2006). As soon as she saw the Castle the first thing she told us was: "How can I visit Cinderella's Castle if I don't have a princess' dress?". You can be sure we made a beeline to the nearest Main Street store and bought her one right there. She went right to the castle and after that to the Carousel, where she rode a white horse with Royal elegance, waving just so to everyone.
That made her trip. Everywhere she went the CMs greeted her with a "Hi, Princess!". That dress sure got a lot of mileage. Best buy we ever did.
As a side note, Disney can be a tiring experience for a three year old. Lots of walking, a constant "bombardment" of sights and sounds, etc. Just be sure to don't "push" her too much. rest, rent a stroller, stick to her eating hours (if possible), etc .
I'm sure she'll have a great time. And for her parents it will be an unforgettable experience.