The PLC does not support the RTS control line, so if you are using COMM3 to talk to an Auto485, the Auto485 must be in Auto mode. The LEDs will not be very brightly lit unless there are continuous data exchanges between the PLC and the device via the COMM3 port. If the device is not responding to command from the PLC, then only red LED will be lit and green LED should be off.
The fact that you saw both red and green LED brightly lead may suggest a wrong connection. Note that male-to-male gender changer may not change the device type correctly (DCE or DTE). I think you need a null modem or make a cable yourself such that:
Auto485 Device
2 3
3 2
5 5
You can test if the Auto485 works by using it for programming/online monitoring via a PC. Of course you can only do so if the PLC is not sending data out of the COMM3 port. You can stop the PLC from using COMM3 port by turning ON DIP switch #4 and reset. For COMM3, after reset with DIP switch #4 on, the default settings is 38,400,8,1,n and you can then use TLServer to talk to the PLC. If you can use TLServer to talk to the PLC and you are able to perform online monitoring of the PLC, then Auto485 is functioning perfectly.
There were problems in your settings as stated in the original reply to your first post. Do modify your settings and it should work.