My understanding is that the Z5 desktop client supports 3-way video calls as you are requesting.
Alternatively, if you're fond of another VRS provider's videophone client, what you are probably looking for is a Multi-Conference Unit (MCU) room where multiple video calls are turned into a single video call stream. ZVRS has offered such a room to BVS customers for years now, but I'm not sure of the availability for non-business users.
I've been out of the VRS industry for almost 2 years now, so I may be a bit out of touch. There may be other videophone clients that support 3-way calling as well, I may just not be aware of them.
