Info from Wikipedia
The gameplay of Halo 3 builds upon the previous iterations of the franchise; it is a first-person shooter which takes place on foot, but also includes segments focused on vehicular combat. The balance of weapons and objects in the game was adjusted to better adhere to what Bungie Studios Multiplayer Designer Lars Bakken describes as the "Golden Triangle of Halo". These are "weapons, grenades, and melee", all three of which are normally available to a player. Halo 3 also contains the ability to dual-wield, where a player forgoes both grenades and melee attacks in favor of the combined firepower of two weapons simultaneously. Most weapons available in previous installments of the series return with minor cosmetic and power alterations. Unlike previous installments, all weapons a player is carrying are visible; weapons not in use are holstered or slung across the player's back. Halo 3 also introduced "support weapons", which are exceptionally large, powerful and cumbersome two-handed weapons such as machine gun turrets and flamethrowers. These weapons drastically limit the player's normal combat options and slow them significantly. In return, they offer greatly increased firepower. In addition to new weapons, Halo 3 added a new class of usable items called Equipment. These items have various effects and functions, ranging from defensive screens to shield regeneration and enemy confusion.
Halo 3 also features a strong vehicular component, with new vehicles never before seen added in the third installment of the series. Most vehicles can be used by the player, but some are A.I.-controlled. Certain vehicles usable in campaign are not available by default in multiplayer due to balance concerns; similarly, some vehicles are only found in multiplayer.
