Bungie hasn't said so but it certainly seems likely that they would stick with the Easy, Normal, Heroic and Legendary modes from Halo 1 and Halo 2. UPDATE: Whether Halo 3 will have any difficulty-modifying skulls, such as the Mythic and Thunderstorm skulls in Halo 2, is unknown. Although most skulls have some effect on difficulty, those two in particular do so, as Thunderstorm changes all enemies to specops, and some players refer to "Mythic" as a separate difficulty level, even though it is not accessible from the menu as such.