From Halopedia, the Halo wiki
Halo 3 features an extensive multiplayer component, playable with up to four players via splitscreen and up to sixteen players over Xbox LIVE. The primary multiplayer component is played through online Matchmaking, though players are also able to create and edit custom map variants in Forge and play with custom rulesets and minigames in the Custom Games lobby. The game also features the Theater, allowing players to replay previous matches with the ability to pause, fast-forward, re-wind and free-cam.
Halo 3 allows players to battle each other in a variety of team-based and free-for-all multiplayer modes via the use of Matchmaking. In online multiplayer, players compete to win gamemodes across a varity of maps and modes, relying on skill to win. Matchmaking play allows players to increase their rank and unlock new armor and achievements.
Forge is a new addition to the Halo games, allowing players to place, edit and move objects placed around the various multiplayer levels. These changes can range from weapon and item placement tweaks to entire structures and minigames to art and aesthetic creations!
Custom Games are an additional component of Halo 3 multiplayer, allowing players to play custom map and gametype combinations with up to 16 players.
Halo 3 ships with 11 multiplayer maps on the disc, with a further 13 added across the DLC. All Halo 3 DLC is now permanently free on the Xbox LIVE marketplace, and is included in the Halo 3 component of Halo: The Master Chief Collection and the Halo 3: Mythic disc included with Halo 3: ODST.
Halo 3 supports a wide range of gametypes, allowing for a variety of playstyles ranging from small-team competitive deathmatch to large-scale capture the flag with vehicles. Custom game settings allow players to customise these game settings even further, allowing for the creation of unique minigames and game variants.