[Uncle Peter]

I would be inclined to setup three mailboxes with the desired primary smtp address, ditch the Outlook client and use OWA to access to each mailbox. When it comes to managing access to multiple mailboxes Outlook is about as useful as a chocolate fireguard. One of the main problems being that your sent items are deposited in the primary mailbox for your profile (unless you start faffing about with rules), you could also forget to populate the TO field and send from the wrong account.