The Elecraft rigs have flexible interfaces for alternate microphone settings. However, there is only the single TRRS plug for a Microphone on the KX3. You should be able to wire up your own microphone to the diagram I have included here (extracted from the User Guide).
It's not necessary to turn it off while it is not connected. Zero dBm is a power level of only 1 milliwatt, and no device will be damaged by that power level.
If it were, the manual would have said so.
In the end I decided to build a simple adapter. Most handheld microphones available at cheap prices have two-jack "Kenwood compatible" interface. They have 3.5mm TRS jack and 2.5mm TRS jack.
The sleeve on 2.5mm jack provides ground contact. PTT contact is on 3.5mm jack sleeve and microphone contact is on 3.5mm ring.
I bought two jack plugs, soldered the ...