My 3 year old is playing up lately too. I think is just the age. They realise they can push you & test their boundaries & will do anything to see if you are true to your word.
All the advise I can give is to stick to it & be consistant. Try to always remain calm too. Flying off the handle doesn't help.
They should grow out of it soon. Well they bl00dy better! I'll go mad other wise!
Sorry I can't help much, but just know that you aren't the only one.