Matchbox Twenty frontman Rob Thomas has agreed to serve as a mentor for Team Cee Lo on the upcoming third season of NBC's vocal competition series "The Voice," according to a report from The Hollywood Reporter.  As a mentor on the program, Thomas will meet with contestants and caoch Cee Lo Green, providing his input aimed at maximizing the singers' chances during live performances.  Thomas joins Mary J. Blige, who will help mentor Adam Levine's team, and Michael Buble, who wll mentor Blake Shelton's team.  The show's producers have yet to reveal a mentor for Christina Aguilera's team.  The third season of "The Voice" premieres on September 10th on NBC stations.