So {Meletis Astronomer} says I MAY reveal an enchantment. If I don't do I still have to put those three cards on the bottom of my library? Or just back the way the came off top?
The remaing cards still go on the bottom. It just gives you a chance to put an enchantment from among them into your hand. Regardless of whether you choose to do so or not, the cards go on the bottom afterwards.