You can see a lot of Disney in 3 days, but you will certainly miss a lot of Disney, too.
Epcot has always been a 2-day park to me, MK a full day, MGM a full day, and AK 1/2 to 2/3 of a day. If you plan well you can see an awful lot of stuff in 3 days; just don't expect to see it all! You will have to cut out some stuff.
I am a big Epcot fan, so I'd spend a whole day in Epcot. I'm a movie buff, so I'd spend a full day in MGM. I'm not a fan of many of the attractions in MK, so I'd split a day between MK and AK.
If I had 3 days, my itinerary would be thus:
Day One: Epcot Future World most of the day, with lunch at the Electric Umbrella (quick counter service) and dinner at one of the World Showcase restaurants, then Illuminations.
Day Two: MGM most of the day. Lunch at Backlot Express (another quick counter service), dinner at Brown Derby or Mama Melrose with the Fantasmic dinner package. If MGM closes way early, go to Epcot after Fantasmic ans continue around the World Showcase.
Day Three: AK in the morning. Breakfast at Tusker House (quick but excellent counter service), lunch at Flame Tree BBQ (another quick but excellent counter service). Not long after lunch, hop a bus to MK. Stay till park closing and pick up the highlights - Space, Thunder, and Splash Mts, Buzz, Pirates, Haunted Mansion, Mickey's Philharmagic (DON"T MISS!), maybe Pooh and, if you're brave enough, Small World. If there are fireworks that night, have dinner at 'Ohana in the Polynesian and ask for a table with a view (you will need advance PS to get a dinenr time during the fireworks, they go fast!) After dinner, if it's not too late, back to Epcot. If it is too late, resort hop; explore the lobbies, gift shops, and grounds of the monorail resorts.
Be prepared; this will most likely be an exhausting itinerary, but tons fo fun and very rewarding!