per NORTEL. The 3rd PRI has to be a point to point see below. This is the response from ASK at Nortel
Regarding your inquiry on the following:
In a MICS: I can have 3 PRI's. The first 2 PRI's (in the core) are connected to the public network (DID/DOD). The 3rd PRI (located in Trunk Module #8), can this also be connected to the public network (DID/DOD) or can it only be used as a Point-to-Point circuit tied to another location?
Response:
No, that 3rd PRI can only be used as a point-to-point connection with an ending location.