DH and I did 3 parks in one day last weekend (May 20th) and had a great time. We decided ahead of time which rides we wanted to do at each park. We started the day at AK, as we really wanted to try out Everest - that was in fact the only must do there since we've been there many times. We were sure to be there well before opening and headed straight to Everest. We rode Everest 5 times (combination of standby, fast pass and single rider), Dinosaur, Primeval Whirl, and Pangani Trail. I would say we definitely could have done the Safari and Kali if we hadn't wanted to ride EE so many times! We headed out around 11:15.
Next, we caught the bus over to Epcot. There were were able to do Mission Space twice, Test Track twice, Journey into Imagination, Spaceship Earth, Lunch at the Sunshine Seasons Food Court and a trip over to the World Showcase to check out more of the Flower and Garden Stuff and browse through a few stores. The line for Soarin was too long for us (50 minutes) and the return time of 6pm was past when we were going to move along. I think we left Epcot at about 4.
Finally we ended the day at MK, as it was open the latest that night (11pm). We were able to do Space Mountain twice, Splash Mountain, Big Thunder Mountain twice, Winnie the Pooh, Jungle Cruise, Haunted Mansion, Buzz Lightyear 4 times (twice with no line during Wishes), dinner at Columbia Harbor House and watched most of SpectroMagic before heading out at about 10:45pm.
We were exhausted by the end, but it was very doable and we had a great time.