If you are wanting the end devices to AUTOMATICALLY respond with data without any arduino sketches
then you have to consult the Xbee manual. and use the Xbee config program.
however if you have the ALL the XBEE connected to Arduino or other microprocessors, you have to
send your data in such a way so that the end device knows the message is meant for them as each
xbee has an address.
You should use the AT command set to send out a broadcast /unicast.
Then the other 2 Xbees are also in AT command mode and your Sketch must wait for
the data to come in. When the command is received, the other 2 Xbees send their
response using the AT command set back to the coordinator.
You would need a ring buffer in your coordinator sketch to store the data
as it comes it quite fast. you then check your ring buffer to see if there
is anything to process.
Your solution is in this thread belowhttp://arduino.cc/forum/index.php/topic,104648.0.html
see how people are using it.http://arduino.cc/forum/index.php?topic=61238.0