@KJ5W I am not experienced with those rigs. I hope experienced users do post.
I have seen posted previously that the Xiegu G-90 is standard levels, Elecraft KX3 is RS232 logic levels for CAT control, Yaesu FT-891 is standard levels. So one digirig will not work for all three if you want CAT control for the KX3.
Cables for all three are in the Digirig online store.
73 Constrainted