These two definitions are both right. In the popular T.I. song, What You Kno, T.I. uses a metaphor to bring together the act of sellin cocaine three kilograms (keys) at a time with the act of calling to arrange the deal. When he says "key by the three" he is stating that he has three keys of cocaine and also that he is using his nextel to arrange the deal.
"I gotta key by the three, when i chirp shawty chirp back"
T.I.--What You Know