@ more AA6YQ comments below
+ I suggest that you first configure Commander to reliably switch your K3 between RX and TX using CAT commands.item (1) is tricky since with the K3S, or any K3 with a USB connection, the PTT signals are on the same virtual serial port as the CAT traffic.
The RS-232 and USB virtual serial port RTS and DTR connections on a K3 are effectively in parallel.
@ Thanks, Bill! I should have included an additional step 2.5 in the instructions above:
2.5 For A K3S, or any K3 with a USB connection, On the Configuration window's Ports tab, set the DTR and RTS selectors in the "Primary CAT Serial Port" panel to "Off".