Philharmagic.
Are you looking for suggestions in the other parks as well or only MK?
My 3 year old was very happy making a loop from Small World to the Carousel to Philharmagic. Could have just spent a whole day there...
I know it's not a ride, but I recommend stopping to see the parade, and also catching a dance party - little ones tend to like to move, so those worked great for us. The stage show in front of the castle is fun too.
A lot depends on how she handles dark rides. Buzz Lightyear can be fun, but it's also pretty dark, so some kids handle it better than others. Same goes with Peter Pan. Very cute ride, but it's in the dark.
In Epcot, you can meet different princesses in the different countries, which could be fun. There's also the KidCot stuff - she might like that. Rides she'd probably enjoy include Imagination and Nemo. Turtle Talk with Crush is great.
In HS, I recommend the meet and greet with Woody and Buzz, but there's the Voyage of the Little Mermaid, and the Disney Jr. Show. Toy Story Mania is fun - but at 3 she might not quite get it, and it might be more of a challenge for her.
In AK, see the Festival of the Lion King, and she'll love the Safari. It isn't a ride, but the Boneyard is a lot of fun for kids, giving them a place to play, run around, and they can also do a mini-dig sort of thing.