The 3-prong sockets used by Qantas in their premium cabin with their noise cancelling headphones use the two larger prongs for the audio channels (left and right) and the smaller third prong to supply power to the headphones to operate the active noise cancellation electronics.

So when using your own headphones, you just need to connect to the two audio connections and ignore the third whole in the socket. To adapt a standard 3.5mm stereo headphone plug to the two whole socket, you need an adapter like this one. I carry one of those inside my Shure E3 case.