Yes, you can just create sip trunks to each of the 7100s. So each 7100 would have a sip trunk to the other 2 7100s and then in the trunk group settings adv accept templates, you would match on area code calls to those cities, remember that most users are going to be dialing 1 for a long distance call so you would enter 1-770-NXX-XXXX for example, then you would go into the Trunk Account for that site and then strip the 1 at the bottom under DNIS substitution match 1-770-NXX-XXXX and substitute with 770-NXX-XXXX so when it gets to the other 7100 it will appear as a local call.
More info on networking 7100s can be found here:
Hope that helps.
Thanks for the reply. Works!