The COMM3 port on T100MD2424 works exactly the same as that found on the T100MD888+ PLC.
1) Have you tried using the TLServer to communicate with the PLC via the RS485 port? I am assuming you have some kind of USB to RS485 adapter or an Auto485 adapter that convert your RS232 interface to RS485 which allows you to test the communication with the PLC via the TLServer.
2) If TLServer doesn't work, try to set the DIP switch #4 to ON, then power recycle the PLC. The PLC should start up in the PAUSE mode, which ensure that nothing in your PLC program has modified the COMM (such as SETPROTOCOL 3, x which disables it from responding to host link command)
3) If you can't get serial communication with the PLC via RS485 port even with DIP switch #4, then it is likely that the RS485 driver chip may be damaged and need to be replaced.