David, this is the same problem I am currently facing. With multipoint L2TPv3, you need to provision additional interfaces, on the LAN side of the router, to create the xconnect mesh between all destinations.
R1 (E0/0) ------xconnect to------ R2 Loopback IP with unique VC ID (12, R1 to R2)
R1 (E0/1) ------xconnect to------ R3 Loopback IP with unique VC ID (13, R1 to R3)
R2 (E0/0) ------xconnect to------ R3 Loopback IP with unique VC ID (23, R2 to R3)
From the perspective of R1, you would connect E0/0 and E0/1 into a switch and let spanning tree take over. The same would be true at the other two routers. I’ve got this working with a single vlan right now. You may be able to do this with sub interfaces as well. I don’t have a lab to test with at the moment. Like I said though, I am running into the same scaling problems.