I only started going a little over five years ago, so I can tell you some of what I think is new since then.
Country Bears were already gone when we started going and had become the Winnie the Pooh ride. Buzz Lightyear is in Tomorrowland. The old subs ride has been brought back and made into a Finding Nemo ride. The castle walkthrough had been closed since 2001 or so and has since been reinvented and reopened. Abe Lincoln will be different since it closed for many years and reopened recently with a new animatronic. Captain Eo has returned, displacing Honey I Shrunk the Audience. And, of course, all of DCA.
Three days is good for experiencing most of what the parks have to offer, but you won't see it all. I would parkhop everyday rather than planning one park per day. The entrance gates are right across from each other. With three day tickets, you will also be able to take advantage of early entry to
Disneyland one morning.