HI Steve. This shouldn't be impossible. From my understanding is that there should be 3 calls from the calling T3. So when the Center connects, Left and Right make there call to the far end T3 system, so from VCS perspective its 3 calls and should be able to connect. If you feel customer is going outside network via Expressway, double check search history on VCS E to see what happened with the calls from the left and rigth to see why it failed.
If you want to look at signalling on VCS E, Maint>Diagnostics>Diagnostic Logging and set log level Debug to see if something may be there as well.
The calls may be able to connect, but might be media problem as well, so packet trace at VCS E may prove useful to see if media is being sent to the opposite T3.
Hope this helps.
VR
Patrick