Halo 2 had originally been intended to wrap up the story begun with Combat Evolved, but development difficulties led to a cliffhanger ending.

[3] In addition to weapons, Halo 3 contains a new class of gear called equipment;[4] these items have various effects, ranging from defensive screens to shield regeneration and flares.

Forge enables players to insert and remove game objects, such as weapons, crates, and vehicles into existing multiplayer maps.

[12] Halo 3 offers a form of file sharing, where items such as saved films, screenshots, and custom variants can all be uploaded to Bungie's official website.

Anyone can browse user created content that has been uploaded to Bungie's website and tag it to automatically download to their console next time they sign into Xbox Live on Halo 3.

[16] Local area network or Xbox Live supports up to sixteen players in multiplayer matches, with game modes including variations of deathmatch and Capture the Flag.

In Halo 2, the Gravemind escapes from confinement, invades the Covenant mobile capital city of High Charity, and captures the human AI Cortana.

[9] After the events of the comic tie-in Halo: Uprising,[22] the Master Chief arrives on Earth in east Africa, where he is found by Johnson and the Arbiter.

The Chief clears anti-air Covenant defenses so Hood can lead the last of Earth's ships against the Prophet, but Truth activates the buried artifact, creating a slipspace portal which he and his followers enter.

Gravemind forges a truce with the Chief and Arbiter to stop Truth and defeat the remainder of his army, rescuing Johnson and halting the installations' activation.

The Chief decides to activate this Halo; the ringworld would eliminate the Flood infestation on the Ark while sparing the galaxy at large from destruction.

However, the force of Halo's blast causes the slipspace portal to collapse, resulting in only the front half of Forward Unto Dawn, carrying the Arbiter, making it back to Earth.

If the game is completed at the Legendary difficulty level, the scene continues to show the piece of Forward Unto Dawn drifting towards an unknown planet, setting up the events of Halo 4.

[27] While details of Halo 3's multiplayer were widely disseminated in the sixteen months leading up to the release,[28] the single-player aspect of the storyline was kept relatively secret throughout much of the development to build up interest.

[29] A public beta test of the game's online multiplayer features, as well as saved films and file share, took place four months before the full release.

[47] Voice actors returning to reprise their roles in Halo 3 include Jen Taylor as Cortana, David Scully as Sergeant Johnson and the Elites, Keith David as the Arbiter, Tim Dadabo as 343 Guilty Spark, Ron Perlman as Lord Hood, Robert Davi as Rtas 'Vadum, and Steve Downes as the voice of Master Chief.

[48] Additional voices include celebrity presenter Jonathan Ross,[49] Nathan Fillion, Adam Baldwin, Alan Tudyk, Katee Sackhoff, and John DiMaggio.

[48] Members of the Halo machinima Red vs. Blue (Burnie Burns, Gus Sorola, Matt Hullum, Jason Saldaña, Geoff Ramsey, and Joel Heyman) have cameo roles.

Marketing took the form of stages, including trailers of the game, real-time cinematics, recorded gameplay sequences, pre-rendered CGI, and live action film.

[50] On September 12, 2007, the "Believe" Halo 3 ad campaign, focused on the epic nature of the story and heroism told through dioramas and third party accounts of Master Chief's service, began with the video "Museum" and continued on past the game's release.

PepsiCo announced a new line of soft drink, a variant of Mountain Dew named Game Fuel, branded with the Halo 3 logo and the Master Chief.

Microsoft's Entertainment and Devices division were quoted as being "disappointed" that the Argos leak happened, but stated they had no intention for punishing them for an "honest mistake".

[58] The retail copy was also leaked online over a week before release, with the 6.14 gigabyte file of the game ripped from a disc and downloaded by thousands of people within 24 hours.

[70] A remake of the Halo: Combat Evolved map "Chill Out", titled "Cold Storage", was released as a free download on "Bungie Day", July 7, 2008.

[91] Advertising Age reported that movie studio executives were convinced the release of Halo 3 harmed box office receipts; the week's take was 27% less than the previous year's yield.

[104] Reviewers including Eurogamer's Rob Fahey, GamesRadar's Charlie Barrett, and GameSpot's Jeff Gerstmann felt that the underlying formula of previous Halo games was unchanged, but that this was not a detriment.

[98] "Every type of Halo fan, from the hardcore to the casual to the brand new, will find something to satisfy them in Master Chief's third adventure," Barrett asserted,[6][105] while IGN's Hilary Goldstein referred to Halo 3 as "the most complete game available on any console", specifically stating "the Forge and the replay functionality raise the bar for console shooters so high, it may never be surpassed this generation.

[104] Gerstmann, GameSpy's Gabe Graziani, and Goldstein maintained that the campaign was too short, especially on easier difficulty levels or with three additional players in co-op.

[6][106] Goldstein was highly critical of the eighth level, stating "the penultimate chapter is so bad, just thinking about it puts a rotten taste in my mouth."

[107][108] Goldstein and Steve West of Cinema Blend thought a part of the game's story was lost by not having the Arbiter featuring as prominently as the character was in Halo 2.