On international PE (sold as such) headphones are provided, not Bose, just some over the ear cheap ones.
The headphones provided have the standard one pin connection.

On the seat, it looks that there are three jacks, but there are really only 2, the third one is just a screw.
While there are 2 jacks, you connect the provided headphones (or your own standard ones) to any of the jacks without an adapter and it works fine.

As far as the location, yes, they are in an odd position behind you underneath the armrest. But my second time on PE, I noticed that the armrest pivots on the long side and you can lift it and then there is a clear view of the jacks, power port, etc.