Huh, John you are wrong. For PS-3 Diesels the front coupler will not work with out a rear coupler sensed or a jumper with a resistor in place. F units should come with that jumper.
So you can get a ps-2/3 type coupler and replace the Trail A PS-1 coupler with the PS-2 type and then both will function when you wire it up to the Rear coupler connection on the PS-3 engine. G