I've tried the 3 parks in one day (though they were all Disney theme parks) and would not suggest it. To much time is spent hurrying from one place to another. You would spend even more time travelling to Universal. Depending on how much interest you have in Universal, I would set one day aside for Universal Studios and Islands of Adventure, and go to the 2 Disney theme parks on a different day. It will be much less stressful.
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