I can totally understand where you are at.

My DD2 when through a stage of randomness where nothing worked: time out, having toys removed, even *smacking* shock horror! And that's when I decided it was time for a change of approach and be more pro-active about encouragement rather than re-active to bad behavior (which is definitely what they want - damn 3 year olds are so smart!)

I find that having DH at home over the christmas break throws my DDs out of whack. Things calm down now that he is back at work.

Keep up with the positive reinforcement. It's worth it in the end.