There are 2 parks in universal studios. So you would have to plan in advance whice rides you wont be able to go one as the queues an range from 2-3hours each ride. And getting to the other park takes about 20 min walk if you don't get the Hogwarts express. I would say 2 days minimum
Yes. Three days more than enough. If you do not have children two days will suffice.
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