If my reading between the lines of this is correct, Lionel was at one point providing the 3rd pin (implied free of charge) to anyone needing to use one of their 2 pin connector adapter cables with with the newer 3-pin PH180.
Dave Olson in the same reply said that essentially all the pin does is depress a switch inside the PH connector that turns on the output power. @StevefromPA and others have inserted a stiff piece of rolled up electrical tape to depress the switch (scroll down a few replies in that thread for an example picture).
If that doesn't appeal to you, maybe call Lionel Customer service to ask if they're providing the 3rd pin for free.