Hi Heidi,
The two best restaurants in Walt Disney World to see the Princesses are Cinderella's Royal Table in the Castle in Magic Kingdom and Akershus Royal Banquet Hall in Norway in Epcot. Both meals have similar Princesses but my family have found Akershus to have the best interaction, it's also cheaper and only uses 1 dining plan credit if money is a concern at all.
What I suggest you do is look at both restaurants here on disneyworld.com, you can see Cinderella's Royal Table
HERE and Akershus
HERE. You will find links on both pages to view the menus as this may help with your decision. Personally my children liked breakfast at Akershus the best, I booked the earliest reservation so we could walk through an empty Epcot which made it extra special!
It is most little girls dream to dine with the Princesses in the Castle but as your daughter is only 3 it may be nice to dine at Akershus and keep the Castle for another trip when she'll really appreciate it. Also don't forget you can visit the Princesses in the Hall of Fame tent in Toontown in the Magic Kingdom and they can also be found in their various countries all around World Showcase in Epcot.
Which ever you choose both restaurants get booked up very quickly, you can make reservations up to 90 days in advance either online or by calling 407-939-3463 and I strongly suggest you book at 7am EST when you are 90 days away!
Have a magical trip,
Jo