Is citalopram 20 mg. Good 3 months after expiration date?

Yes-- but. It is still functional-- but if it is that old, it sounds like you are no longer under a doctor's care. It is never a good idea to take medicine unless your doctor knows about it. You might have been placed on new medicines that interact with it, or, at the very least, you need to have someone to call if you have side effects.