Dear K.,
First off RELAX, it's a stage. My oldest son Zach is 3 1/2 and is showing the same behavoir (and has been for the last 6-8 months). I joke that I went to bed one night and my sweet, caring lil man woke up and became a spawn of the devil....LOL
I've tried alot of different things, what has helped for us is structure. I structure his time everyday. We gave him a list on the fridge that shows what to expect. (i.e. Wake up and go potty on the potty, brush teeth, eat breakfast, 30 minutes of reading the books you want, 2 hours of outside time, etc) We have even put the special functions I try and do once a week with him on the schedule (he does play date at the mall on Tues, I take him to the Arcada on Weds (double tokens, double points) if he has been kind to his younger brother, and followed most of the rules.
Structure has done alot for Zach, and curbed 60-70% of the meltdowns. But at this age, there will always be some (I have five younger brothers, the oldest is 7 years younger, the youngest is 18 years younger. I know about raising boys. Being the only girl and the oldest I had to help raise my brothers, and loved every minute after I became older and moved away...LOL)
I hope my info helps. Every child is different, and sometimes you have to try alot of things before you find what works for you and your bundle of joy. But there is hope, just keep trying different things until you find what fits best for your family.