The Tenor have built in gatekeeper.
The gatekeeper is limited in the ammount of gateways that could login at the same time (maximun of four as default), but it should be able to handle all the calls.
You should try to make one of the Tenor the gatekeeper. The three gateways will register to the gatekeeper. You give your partners the I.P. address of the Tenor acting as gatekeeper.
You need to point the gateways, incluiding the one on the same chassis than the gatekeeper to that I.P. address.
I have that configuration working with a A400 and some A200 as well.