This question is a bit like asking what are the three most important parts of a car. Well there is the engine, and the wheels, and the transmission. But then the engine can't work without a gas tank, and a radiator of sorts, .... And so it goes, each part depends on another. It is the same in photography from the initial composition, to the exposure, ... to the final print. Really no one step is any more important than any other. For without a single step in the process you have nothing.