With regard to the history of the Cold War, what's a clear definition of revisionist, post revisionist, the third theory (if there is one)?
I assume that you are asking about these terms in the context of the Cold War and I have edited your question accordingly.
In order to have revisionist history, you have to first have an orthodox history. So orthodox history is the "third" that you are asking about. It came first. It held that the Cold War was caused by the Soviets' desire to expand. Therefore, the Cold War was the fault of the USSR.
Next came revisionist history. This school held that the US was responsible for the Cold War. It said that the US really wanted an empire and were the aggressors in the Cold War. Gar Alperovitz, for example, argues that the US dropped the atomic bombs on Japan to scare the Soviets. It was this sort of aggressive action that led to the tensions of the Cold War.
Post-revisionists like John Lewis Gaddis attempt sort of a synthesis of the orthodox and revisionist points of view. They tend to argue that both sides were in some ways responsible for the start of the Cold War.