Multiple SBS2011 sites and shared SMTP domain

I have a customer who operate 3 sites, all running SBS2011, all currently with seperate SMTP domains

I'd like to use a single (or even combine the current three) shared SMTP domain - i understand I need to use a Non-Authoritative Accepted Domain with (?) internal or external relay  

I'm not quite sure how to proceed with this - Can any of you provide some guidance?