It depends upon whether you have the KIO3B USB interface with internal sound card in your K3s or not.
If you have that, then all you need to use is a single USB A to B cable for both audio and CAT control.
All K3(s) can work with RigPi but if you have only the analog TX RX audio interface on the rear of the K3 then your RigPi will need to have its own sound card.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi