Is this a first trip?
I think 3 days is perfect for a first trip. 1 day for each park; you'll want to try most everything at least once to see what you do and don't like. Then, 1 day to see everything you missed and everything you loved and want to do again.
Then for future trips, you'll have a better idea of what you really like and how much time you'd need to see it all.
Also, if you have HP fans, don't underestimate the amount of time you could end up spending there. Many people can make those sections of the park a full day by themselves. First time to HP, the kids spent 6 hours in Hogsmeade, DA wasn't even open yet. They wanted to ride all the rides multiple times, shop, eat, send owl mail, get wands, explore, and watch the shows.