You definitely, without a doubt, have to do Magic Kingdom, but this is your decision, and I'm going to give you some background information so it will be easier for you to choose. Disney is awesome! (And Yes, you heard right. It's almost impossible to get every ride, show, park, attraction, sight, shop, restaurant, and musical done in one trip. And you'll regret trying to cram everything in one, trust me.)
As the first park ever created in WDW, this is a MUST-DO. Picture thrill rides, calm rides, attractions, sights, shows, audio animatronic attractions, a beautiful castle, and awesome gift shops? Oh yeah, this is the Magic Kingdom. Awesome right? There's a 99% chance this could be your favorite park, it's everyone else's favorite almost. There's nothing more to say really. You just have to go there, no explanation. You MUST!
Epcot stands for Experimental Prototype Community of Tomorrow (or, as the Disney Workers joke, Every Person come out tired) Epcot's icon (like the MK has the Cinderella Castle) is a Geo-sphere, also the Giant Golf Ball due to it's look. At Epcot, you'll experience going green attractions and The countries around the world. You can travel to many countries at Epcot and it feels like you're really there! One minute you're in France, the next you're in England! There is also the classic attraction Soarin' where it feels like you're flying over America. The only thrill here is Test Track, which is a car that goes a whopping 65 mph, and it is the fastest ride in all of WDW parks. If you enjoy eating out, traveling around the world, Going Green Attractions, and a little more then you'll enjoy Epcot.
MGM Hollywood Studios:
There is more thrill at this park. MGM's icon is the Sorcerer Mickey Hat, and the point of this park is all the classic Disney Movies. The Great American Movie Ride will show you many movies and let you feel like you're in the scenes when you see aliens jump out of ceilings and a girl enters a world of color as she soon realizes she's not in Kansas anymore. There are amazing shows to watch at MGM, and a wonderful live performance of Cars actually performing stunts and catching on fire in front of your eyes. There is the Twilight Zone Tower of Terror, where you drop up and down thirteen stories. And there is A rockin roller coaster starring Aerosmith as you speed from 0-60 miles in a matter of seconds. There is the Beauty and the Beast musical too, which is very good from the reviews I've read. At nighttime is the most amazing performance, starring Mickey. As Sorcerer Mickey battles all the Disney Villains, you get sprayed with water, see a lake catch on fire, watch water spray as Disney Movie scenes are being scanned over it, and the whole time ask how did they do that? If this sounds like your theme park, put it on your top three.
This, obviously, is themed on all animals. This park's icon is the tree of life, and in the tree of life is the show It's Tough to be a Bug (which I thought was pretty good). There is a water ride and a roller coaster ride, and a bunch of Animal Exhibits. There is a real safari that you're guaranteed to see animals on. And a mysterious Woman, Divine, who is all covered in vines and moves very slowly. She hides among the trees at Animal Kingdom and is hard to spot. Seeing her is a rare occasion (I myself have only spotted her once) definite snapshot.
If you like Animals, Animal Kingdom is the park for you. All there is is animals! Everything there is based on them! Even the musical shows (which are Finding Nemo and the Lion King)!
Honestly, if I were going to WDW for three days, I would choose Magic Kingdom, Epcot, and Animal Kingdom. MGM Hollywood Studios is finished in a matter of hours for my family, and then you're stuck there the rest of the day eating, shopping, and waiting for Fantasmic to start.
However, if you have a park hopper, you can get all the parks done! (ok, let me reiterate that)
Magic Kingdom you can never really get done in one day, bu most of it will get done. For Epcot, the same thing, the only attractions I've missed once when I was at Epcot was Ellen's Energy Adventure and The three Caballeros, and we were only there for maybe five hours. With a park hopper, I switched from Epcot and went to MGM Hollywood studios, and we saw Fantasmic and stuff.
MGM is over in a matter of hours in my experiences, and Animal Kingdom can take all day like the Magic Kingdom as well.
So, my decision would be with a park hopper:
But if you guys don't have a park hopper, my choice would be:
But it's really your choice with your family to decide, and don't worry, I've gone to Disney before for three days and you get mostly all the parks done. I hope you all have a wonderful trip and decide what parks you guys will go to!
Hope this answer helped!