How to get all you iMessage devices working in unison
The secret is that the CALLER ID has to be the same on all devices and it CANNOT be a phone number (i.e. your Mac doesn't have a phone#). This is not the caller ID that people you call would see.
It doesn't have to be your apple ID email either, it just has to be an email address and it has to be the same on all your devices.
On your iPad & iPhone go into Settings.
Select Messages.
Tap on Receive At
Tap Add another email and enter the email you are going to use.
Then go down to Caller ID and tap on that email address so it is checked.
On the Mac open the Messages app, select preferences under the Messages menu.
Click on Accounts.
Left column should have Bonjour and your Apple ID. Click on the Apple ID
where it says, you can be reached for messages at:
Click on the + sign and type in the email your going to use.
Press return.
On the Caller ID selection, pull down the menu and select that email address.
Voila, you are done.