We like to spent two full days in each park, but three days should be fine. Just make sure to hit the major attractions first, which would be The Mummy, Men in Black, Shrek and ET at Universal Studios, and Spiderman, The Hulk, Dueling Dragons and Jurassic Park at Islands of Adventure. You can spend one day at each park and use your third day to visit both parks and re-ride your favorite attractions.
Have fun!