P.A.
Children at this age are learning so much. It is important to give them two choices. Usually both are what you would like them to do. A win situation for you both. Children at three are trying to take in A lot of information. No is a word they see has power. So they use it. Usually if you respond positively and distract them by saying, come and let me show you this fun thing, they forget they have actually said "no." I remember the story of the Mother who spanked her child because after the Mother told her not to go to the corner of the street, she went anyway. After the spanking occurred, through the little girls tears she said, "Mommy what's a corner?"
Children are learning and our world is big to them. It is important to listen, be patient and divert while they are young. They will learn through us and through our patience.