Three year olds are still exploring limits. Parents are the one's who put them in place, and then reinforce them with their child. He understands the word 'no', he just continues to do what most 3 year olds do, and test your resolve when you set a limit. Children are interestingly resolute when they want something their own way. I suggest that when you set a limit, make sure you can live with the ensuing tantrums because allowing them to wear you down and give in is just going to make this a monster problem to handle as the child gets older. For some terrific tips and great insight get the book "How to talk so kids will listen and listen so kids will talk." Invaluable, easy reading and full of great insight on various ages and the human psyche. Good luck to you!