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 S3 (Stock, rooted TouchWiz)
Samsung Galaxy Tab 3 7.0 (non-rooted, stock)
Samsung Galaxy Tab 2 10.1 (obsolete, slow, rooted)
Samsung Galaxy Note 10.1 2014 Edition