Yep, I have a princess lover too! Things to do include (will be repeating some of the above):
1. BBB -- I think 3 is the youngest age so that could work.
2. Character meals -- Ariel's is a no brainer but sometimes it looks like one or more of the princes or princesses appear at Goofy's but that is more of a crap shoot.
3. Meet and greets -- Princess Fantasy Faire or whatever it is called in Fantasy Land where you meet 3 princesses (they rotate out so no guaranty who you will meet even if you can see which ones they are); Rapunzel M&G; Merida M&G and while not princess, along those lines is a special M&G with Tinker Bell and her friends (Usually 1 or 2 others, not guaranty who they will be)
4. Rides -- Little Mermaid in DCA; Snow White's Scary Adventure (note might be too scary for your 3YO so worth a youtube to verify); Carousel; walk through Sleeping Beauty's castle; Storybook Canal which has miniatures of the castles and locales for many of the princess movies.
5. The etc. -- parades usually feature one or more princesses. Fantasmic and World of Color both feature princesses as well. Snow White's wishing well on the side of the castle. Characters often pop up there so worth a visit. If your girls are into Princess Leia at all, there is a chance to go hero Star Wars at the Jedi Training Academy. I think the minimum age is 4 but I would swear they've picked 3YOs judging by some youtubes. Merchandising you can find lots of places and particularly in Fantasy Land.