Which studio developed Halo 3 for the Xbox 360?

Bungie developed Halo 3 for the Xbox 360.

Released in September 2007, Halo 3 concluded the original trilogy's central conflict between the United Nations Space Command, the Covenant and the Flood. Bungie built the game as a direct sequel to Halo 2, with the story focusing again on Master Chief and the Arbiter.

Halo 3 was published by Microsoft Game Studios and became one of the Xbox 360's major system sellers. Its campaign supported cooperative play, while its competitive multiplayer and Forge editing mode became especially influential features.

343 Industries is often confused with Bungie because it later became the principal studio for the Halo series. Microsoft created 343 Industries after Bungie's separation from the company; 343 subsequently developed Halo 4 and later mainline entries. Bungie, however, was the developer credited for Halo 3.