I have found the schematics on the Elecraft site. http://www.elecraft.com/manual/E740323%20K3s%20Schematic%20Files.pdf
On the right lower side of the drawing of the KIO3B Digital I/O Module is a pin table how to connect the serial port RJ-45 to the P3.
Pin table is called: J24 Connection to P3 using P3 cable (PC via USB)
With an old Cat5 cable and some SubD 9pin connectors from the junkbox I have made a CBLP3Y for free.
I have no problems buying parts if that is necessary. But in this case, it is cheaper and gives satisfaction after making this cable by myself.