From Halopedia, the Halo wiki
Between 2007 and 2010, Halo 3 had five sets of downloadable content released in the form of multiplayer map packs. These map packs (with the exception of Cold Storage) released for 800 Microsoft Points, though as of 2017 have been made permanently free downloads for Halo 3 players on Xbox 360 and via backward-compatibility. All DLC content is additionally included in the Halo 3: Mythic disc released with Halo 3: ODST, and included in the Halo 3 component of Halo: The Master Chief Collection.
The Heroic Map Pack was released on December 11, 2007 and consists of three new maps; Foundry, Rat's Nest and Standoff. Foundry emphasises an expansion of the Forge mode, while Rat's Nest and Standoff focus on mid-large team action with an emphasis on vehiclular combat.
The Mythic Map Pack was released in 2009 and was included in the limited edition release of Halo Wars and later released on April 9, 2009 for all players. Mythic includes three all-new maps, Assembly, Orbital and Sandbox.
Released in celebration of Bungie Day 2008, Cold Storage is a remake of the map Chill Out from Halo: Combat Evolved. Cold Storage was released for all players for free as a single map download on July 7, 2008.
The Legendary Map Pack was the second map pack released for Halo 3, released on April 15, 2008. It consists of three new maps; Avalanche, Blackout and Ghost Town. Avalanche is a massive big-team map and a remake of Combat Evolved's Sidewinder, while Blackout serves as a small-team remake of Halo 2's Lockout. Ghost Town serves as a small-mid sized map set in a water processing plant deep within the jungles of Kenya.
The Mythic II Map Pack is the second part of the Mythic map set, initially bundled with the Halo 3: Mythic disc and later released for all players on February 2, 2010. As the last map pack released for Halo 3, the map contains three final maps; a remake of Halo 2's Midship now-named Heretic and two new maps Citadel and Longshore.