Each of these devices must have an idividual calling id. As I like to explain it, if all of the calling id's are the same, it's like pickping up a telephone and dialing your own number -- you get a busy signal.
For the mac and ipad you need to associate a different, individual e-mail to each of them. These e-mails need to be verifiable. You can obtain e-mail addresses from popular providers such as yahoo, google, etc.
For the iphone you can simply use its assigned phone number as its Facetime id. Please take a look at the following link which can help you to add e-mail addresses to your Apple id.