Whether you use two eggs or three is up to you. I find that a three egg omelette is too much for me. Whisk the eggs with about a tablespoon of water. Do not use milk. Milk changes the consistency and it won't work as well. Water makes the egg mixture more supple and easier to work with and it won't change the flavor.
Melt a tablespoon of butter in the frying pan and have your egg mixture ready: