Normal 3yr old behavior?

My 3 yr old defies me constantly, no matter what I do to punish him he doesn't care. He breaks things on purpose and is completely destructive. I'm at a complete loss. The pediatrician says he's just a normal toddler but I just can't imagine all toddlers act like this. Any advice or tips? How does your 3yr olds act? Am I the only one?!!