Halo 3, the final video game in the Halo trilogy, is an upcoming first-person shooter by Bungie Studios for the Xbox 360. An official announcement on the developer's website states that Halo 3 will end the current story arc of the Halo trilogy. A public beta test of the multiplayer component of the game occurred between May 16, 2007 and June 10, 2007. The final version of the game is due to be released on September 25, 2007 in the United States, Brazil, Canada, Australia, India, and Singapore; September 26, 2007 in Europe; and September 27, 2007 in Japan. On August 29, 2007, Bungie officially stated that Halo 3 went gold.
(source:http://en.wikipedia.org/wiki/Halo_3#Critical_reception)

Watch the Halo 2 vs Halo 3 trailer at the link below: