Just looked at the extension cord I carry with me on all my trips, and used extensively in the roomettes last winter. It is three-prong.
I just love this cord because the low-profile plug keeps it out of the way. It just happens to be offset in the proper direction to lay against the window behind the tray table in Superliner roomettes. And it has three receptacles for my electronics and phone.
They are three prongs on Superliners. If you want proof, head over to the Virtual Walk Through page here http://www.kaemena360.com/360/AmtrakSuperlinerTour/, then go inside one of the roomettes and you can zoom in on the plug and see that it's three.