You need the the 2 pin convertor to get stereo.
I'm not sure why the 3 pin set-up. maybe to dissuade people from pinching them?.
The Bose headphones on AA are two pin , altho those are collected before the end of the flight on their F service.
The new Airbuses on CX have headphones that are hardwired into the seat which makes it annoying for those who prefer the ear-bud
Sony's (like me)