Buy a couple of 25-way D connectors and a length of three core cable that suits your needs. Solder an individual core to pins 2, 3 and 7 of one of the 25-way D plugs - we'll call it Plug A. The next bit is the tricky bit. The cores still requiring soldering should be soldered so: solder the core on Pin 2 of Plug A to pin 3 of Plug B. Pin 3's core on Plug A should be soldered on to pin 2 of Plug B. Pin 7 on Plug A solders on to pin 7 of Plug B.