The Elecraft KX3 can be remotely powered up by applying 5v to Ring 1 relative to Ring 2 on the Mic jack momentarily. RemoteTx can do this by connecting a modified control relay to the RPi on pin 2(5V), pin 34(GND), and pin 36(GPIO 16).
With this relay installed, the KX3 can be turned on by clicking the ON button in the RemoteTx Radio tab.
After clicking the ON button the relay put 5v from the RPi on the output terminal for 1 second which will turn the radio on. The relay output terminal needs to be connected to Ring 1 of the KX3 Mic jack. The RemoteTx control interface will load within about 3 seconds.
See figure below:
Clicking OFF will turn off the radio and close the KX3 control interface.
Relay jumper modification:
KX3 Mic schematic (from Elecraft Support):
5v Power On Videos: