Water is the main solvent of living beings and it is necessary practically for all biochemical reactions, including as reagent of photosynthesis. Many properties of water are very important for life.
Carbon is the main chemical element of organic molecules; carbon dioxide is also reagent of photosynthesis and product of the energetic metabolism of living beings.
Nitrogen is a fundamental chemical element of amino acids, the building blocks of proteins that in their turn are the main functional molecules of living beings; nitrogen is also part of the nucleic acid molecules, the basis of reproduction, heredity and protein synthesis.