When you run dmdext from the prompt with the parameters you specified, it will give you i formation. Info is stated if the pindmd3 is found on what port. It also should end with something like "waiting for frames". If this is the case, launch FX3 and it should shows that frames are send to your physical device.
A Teensy board is used for adding addressable leds. On my cab the Teensy board is a bit the same as the pindmd3, and change the communication port from com3 to com5. I needed to change that. But the info can be found how I described above. It will show your device and the com port.
dmdext mirror --source=pinballfx3 --no-virtual
This should be the correct command. You can also add to exit dmdext when process is finished.
And make sure tou have the version from begin November 2019 from dmdext !