Is there a reason why 3 year olds say no and then say yes but continue to fuss afterwards? how does one handle this behavior?

Patience: In their 1st year they were the king/queen & you responded to cries & basically fulfilled all their requests. As they begin to walk/develop independence they still expect to be the boss which by 3 is assisted by language. They are confused by the change in status, are still self centered & dealing with need to be independent but still dependent on others. It is a phase they pass through over time.