Nitrogen is the most important of these three.
Plants use it to build proteins, enzymes, chlorophyll, and hormones.
Adequate nitrogen gives a plant healthy green leaves and strong growth. Too little nitrogen results in a pale, slow-growing plant. Too much can give you a plant that's succulent and overstimulated and attractive to pests and disease. Nitrogen is always on the move through soil, atmosphere, and living things and can be rapidly leached away in irrigation water. Therefore, it's the element most likely to be in short supply.