Connecting multiple RS485 together
It is possible you have too many resistors on the RS485 bus. From the RS485 wikipedia page there is this quote:  


  Ideally, the two ends of the cable will have a termination resistor
  connected across the two wires.


This may explain why you can use 2 of your RS485 interfaces but not 3.  Consider removing the resistors from the RS485 board in the middle if possible.