I’m trying to do the same thing right now. Did you ever figure it out?
My best guess is to use a meetme room. First, transfer parties A and B into the meetme room as shown here voip-info.org/wiki/index.php … n+Redirect
Then dial the next person in and drop them into the meetme room.
To do the first transfer, I need the channel for both parties A and B. I know one channel, it’s going to be SIP/EXTEN, but I can’t figure out how to get the other channel.