We just returned a few days ago with our 3 year old (and almost 5 year old). Fun was had by all! We learned that you have to take things on the kids' schedule. Lots of posts here stress the importance of rope drop. That didn't really work for our family, as my 3 year old woke up after 8 most days, needed time to wake up, get dressed, eat some food etc. before we could go out. For this reason, we ended up not doing any early breakfasts. We also needed to eat dinner at a normal hour and get her to bed at her normal bedtime to avoid tiring her out. Also, a stroller is very important and your cousin needs to decide whether to rent one or bring one (I'd bring one because it is helpful in the airport).
As for research, I found the Unofficial Guide to be helpful but the DIS was most helpful. If she likes princesses, I'd definitely do breakfast at Akershus. We did a 10:30 breakfast and my girls loved it. That was the only character meal that we did. My 3 year old collected autographs and really "got" that. She liked meeting all the characters that she saw milling about.
As a first ride, we did Dumbo. Luckily the line was only 20 minutes at 3:30 PM. We were able to walk on to similar rides - Aladdin and Triceratop Spin were both walk ons when we did them. My 3 year old really enjoyed the teacups. Philharmagic was a HUGE hit too. I would say that Fantasyland and toon town are "must do's." The 3 year old may be afraid of indoor rides, so I'd try something like It's a small world first that is not remotely scary and then work your way up to rides with scarier themes/characters. We did Pooh first and my 3 year old was fine but my 4 year old was terrified by the dream sequence! After that she refused to go on any indoor rides for the rest of the day and for much of the next day.
Also, prepare to stop for shows and parades. Fireworks are nice, but if you have to stay up late for them, be prepared for the 3 year old to sleep in the next day. Also, be prepared for the 3 year old to be afraid of fireworks. We didn't have this issue, but many are scared.
The other thing that we did was dress up our girls in character clothes. I got shirts and outfits on etsy and ebay and my DDs were so excited to have special and unique clothing. People fussed over them and the characters always pointed it out.
A 3 year old will likely be too short for many rides. My 3 year old is 39.5 inches and skipped out on a lot of things. If she is traveling with another person, it may make sense to use fastpasses if possible rather than baby swap so that the 3 year old won't be disappointed to knowingly skip a ride. We avoided meltdowns this way. We are a family of 4 and we got 4 fastpasses for rides with height restrictions. Then DH would take my 4 year old. If she liked the ride then I'd use the other two fast passes and ride with my 4 year old while DH hung out with the 3 year old.
Bottom line - every kid is different. But be prepared to do things on the 3 year old's pace and not on the pace you might want.