Bungie has revealed the final three new multiplayer maps that will be included with Halo 3: ODST this September. They're called Heretic, Citadel and Longshore and you can see screenshots in our brand new gallery.
Heretic is in fact a new version of Halo 2's Midship map, set on the Covenant ship the Pious Inquisitor. Citadel is a confined, symmetrical room in a Forerunner stronghold with two opposing bases and little cover. Longshore is an Old Mombasa port area designed for complex, large scale combat, bristling with industrial buildings and elevated walkways.
Halo 3: ODST will be released for Xbox 360 on 22nd September, and includes the new Firefight co-op mode and all Halo 3 multiplayer maps to date, alongside an entirely new campaign starring the Orbital Drop Shock Troopers of the title. More at the ODST gamepage.