If "3" is the external number you can do this a number of ways, one is if your length of digits for mboxes is the same as as the abbr. dial you could put that in there, another way is to put an extension in that is call forwarded external to the abbrev dial number.
To tamdem trunks from the mail, the mail cos must have trunk to trunk capabilites and external call forward, also I would check in form 4 for "incoming to outgoing call forward" option 21 is enabled. This works good as I have done this many times.