Green Day frontman Billie Joe Armstrong has reportedly signed on to become a mentor for the upcoming third season of NBC's vocal competition show "The Voice."  The news was announced in a Tweet posted Thursday night by the show's producers.  It read:  "Welcome Billie Joe (@BJAofficial) to #TeamXtina!  We are ready to rock out in Season 3."  Armstrong will apparently mentor the singers on Christina Aguilera's team, and was the last mentor to be revealed.  Matchbox Twenty frontman Rob Thomas will mentor Cee Lo Green's team, Mary J. Blige will help out with Adam Levine's team and Michael Buble will provide an assist for Blake Shelton's team.  The third season of "The Voice" is set to premiere September 10th.