Three lines means stereo plus microphone. It could just be the headset jack doesn't offer a provision for microphone and relies entirely on Bluetooth for external headsets. Could also be a setting in the phone itself for headsets.
Samsung Galaxy Precedent (rooted, AreaRom 3.4 , sd-ext w/ 3.65GB internal space)
Kindle Fire (1st Gen, rooted, AOKP JB 4.1, Nova Launcher)
Samsung Galaxy SIII 4G LTE (rooted, Carbon ROM)