Most airlines use a 'Twin 3.5mm Mono Socket', which you can get adapters for. I have a set of Sony Noise Cancelling earphones with an adapter for this, and the noise cancelling removes the sounds of the aircraft drone and air con. Well worth it, and I think they cost me about 25 pounds.
Aircraft with the three pins, tends to be for noise cancelling headsets, which airlines such as Etihad provide in economy (very good headets), wheras most airlines only offer these in business.
