The new maps are included with the Halo 3 Achievement where you have to obtain all the Mythic Skulls.
Heretic.
Heretic returns players to familiar ground as the Halo 2 arena "Midship" makes its long-rumored debut in Halo 3. The stark interior of the Pious Inquisitor might appear austere on first glance, but Heretic's symmetrical spaces require players to execute complex strategies in order to outplay their opponents.
Citadel.
Citadel may be a long abandoned Forerunner stronghold, but its small, symmetrical confines are far from secure. Two opposing bases offer little safety for players looking for solace. Instead, Citadel forces small squads to stay in constant communication, keep their heads on a swivel, and be prepared to adjust combat tactics in close quarters.
Longshore.
Longshore lacks the commercial ships that once made it a lucrative port for Old Mombasa, but it still bustles with activity. Designed for large scale combat and well suited for smaller affairs, Longshore's multitude of industrial buildings, elevated walks, and open inlets conspire to create a complex battleground tailored for multiple combat scenarios.
Original source:
IGN: http://uk.xbox360.ign.com/articles/100/1001564p1.html
