Hi there
I will answer this question for you right now.
SX20 by default only allow point to point connections, this means that a third videoconference device (pc or device) is not allow.
For a third device to connect you will need a multipoint license:
LIC-SX20-MS
We have this model in here, so we have done all this testing, we purchased the multipoint license, now we can connect up to 4 devices I beleive.
Multipoint license also has a limit.
Best regards