3 metal sections i.e. 3 connections. Serial port connector on the other end would point to a TX, RX and GND as the three signals.

So it's for a device that is probably fairly small (hence the 3.5mm connector) that requires an RS232 serial connection (and thus low data rate) for config or setup.

Could be anything.
Old digital camera (as mentioned, before USB got big)
GPS

It won't be for large amounts of data though.