In my setup, you'd have something like this:
site A ERL
eth0 -> WAN
eth1 -> AF to site B
eth2 -> SiteA LAN
siteB ERL
eth0 -> AF to siteA
eth1 -> AF to siteB
eth2 -> SiteB LAN
siteC ERL
eth0 -> AF to siteB
eth1 -> SiteC LAN
Bear in mind that this is just a quick mockup, and is really only intended to be illustrative.
