I am trying to set up three different macace.net email addresses to enable sending from the different addressess but I now have three seperate Inboxes, which I don't want, and any received email goes into all three Inboxes.
So all of these are from the same server with the same server SMTP and POP or IMAP credentials, and you want three separate email addresses to be available when sending, and you are using Apple Mail? If so, then get rid of all but one of the server Account specifications, and (when you configure that account) place the three addresses in the Email Address entry in the Account Information section in the remaining server setup, specified as a comma-separated list.
Now bring up a Compose Message window (as if you were sending a message) and select the three horizonal bars and customize the header fields; make sure you get a From field, which should now have a pop-up to select the return address for the particular message. (In the Compose window, t's the button with three-horizontal bars and a disclosure triangle to the lower left within the header portion of the message compose window on Lion; don't have a Mountain Lion to check that detail right now, but I'd be surprised if that changed.)