Three parks in three days back to back will be very tiring, especially if you're going to try and see as much as possible every day, so start your stamina and strength training now. Seriously, if you're not in the habit of doing a lot of walking I'd start getting in shape now so you don't wreck your feet and leave exhausted. You won't be able to see and do everything in each park in a single day, so you'll need to do some research and planning about which attractions and shows are must sees for you. MK has the most attractions, Epcot has the most walking, AK will be the most relaxed of the three so I'd save that park for your third day.