If you think you will return I'd buy seven day passes. Disney tickets are good forever. With a seven day pass you also get 4 additional forever visits to Pleasure Island, the waterparks or I think mini-golf.
However if you aren't sure I'd buy the 4 day hopper passes (that's the minimum). The ability to be able to go to any park on any given day is invaluable. Single day passes limit you to a single park...and since you will only have 3 days to visit the parks you need to be able to hop. And you will be powerhopping. Three days is the minimum you need to be able to see most things in the parks alone. Be sure to only visit Epcot after 5pm to get the most out of your time if you plan to see as much as you can of the other parks.
If you are looking to relax then buy the single tickets. Personally I'd buy Epcot, MK and MGM...in that order...as adults. Other people would probably choose Magic Kingdom first. Animal Kingdom I'd leave for another trip without a hopper.
My personal powerhopping plan would be: Magic Kingdom from opening until 2 or 3. Rest for 2 hours in the hotel room. Go to Epcot until 9.
Repeat the next day for MGM & Epcot...you will not get to see Fantasmic.
Repeat the last day for Animal Kingdom...use the evening hours to hit everything you didn't see. Or skip Animal Kingdom and use the entire day to fill in as much of you can of what you didn't see. Try to make Fantasmic this day.
You will have to pass on a lot of shows so I highly recommend purchasing a guide book or going to
www.wdwig.com for help in formulating a plan of attack.
Your best bet would be to add a day to your trip.