I called it the "terrific trying/doing three" ... I found if I put a POSTIVE spin on the horrible "names" everyone gives the ages it reminds me that I am here to guide and raise my child instead of blaming the age she is in for behavior.
With that we are working on how to properly express oneself when mad, frustrated and upset... tantrums, yelling and hitting are not ok... instead use one's words nicely to express how you feel. We also say if you do not feel like talking but are mad/frustrated it is ok to go sit in ones room for awhile to calm down.
Our daughter is starting to see that the world does not center around her (mentally they start to develop the realization of a bigger world), and it is tough for a child to switch their minds from me to others so we talk about that when the occasion calls for it.
Your child will want to do more for herself, as long as it is safe let her or some how invole her if possible. This helps in learning and it does cut down on tantrums because they will know you let them help/do things but when you say no that there is a reason (it also helps to tell the reason, you do not need to tell them the reason all the time but it helps them understand why they can not do something).
Being consistent in whatever you do (punishment, eating, everything) it will help a lot because any kid is comforted by a schedule and consistency (obviously as long as it is not hurtful or puts them in harms way).