It "kind of works" but you will loose quite some K3 functionality.
For one, the "AFX spatial sound" feature is only available via the headphone / speaker output, not via line-out. You will lose AFX if you interface using the line ports.
Secondly, if you have the subreceiver option, note that there is a hard relationship between these receivers (main/sub) and the line out port.
If you switch off your sub receiver, the K3 will drive both audio channels (if you're using the headphone port, or of you are using the speaker port and have set the number of speakers correctly).
Not so for the line out port: one "audio channel" will go silent.
The way I've "solved" this is to use the headphone port on the back and switching the internal speak on and off using the CONFIG:SPKR+PH setting, which i have assigned to one of the programmable buttons on the front: press the button and the setting toggles and switches the radio speaker on and off.
I used another programmable button to access the MENU:MIC SEL menu. This way I can switch between the mic port on front (when I operate at the radio location) or the mic port on the back (when I operate via remoterig).
I had to play with the gain settings to make the headphone port work.
73, Geert Jan