Season Three ( Book 3 : Fire ) of Avatar : The Last Airbender , an American animated television series on Nickelodeon , first showed its 21 episodes on television from 21 September , 2007 to 19 July , 2008 . Season Three ( Book 3 : Fire ) of Avatar : The Last Airbender , an American animated television series on Nickelodeon , first aired its 21 episodes from September 21 , 2007 to July 19 , 2008 .