just make sure that one is a coordinator and that the other two have associated with that coordinator. Other than that, the default configuration of the XBee S2 Zigbee modules is Point to multi-point with the router and end devices pointing to the coordinator, and the coordinator is in Broadcast to all other nodes at 900 baud, 8 data bits, no parity and 1 stop bit using transparent mode. That means, Data in = data out.