3 Xbee Communictaion : A to B and B to C
If you don't have a specific protocol requirement here is a suggestion:


By using XCTU app from Digi, flash 802.15.4 firmware into all xbee modules. 
Set NODE A as coordinator and others as ENDPOINT.
Set up a mesh network by channel settings.
Now set the NMEA source's (which is NODE A) destination address to broadcast.


When the system starts to work, both NODE B and NODE C will receive the NMEA data sent from NODE A.