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", which are available to a player in most situations. Halo 3 contains the ability to dual-wield, where a player forgoes 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, the player's secondary weapon is visible on their player model, holstered or slung across the player's back. Halo 3 introduces "support weapons", which are exceptionally large, powerful, and cumbersome two-handed weapons which drastically limit the player's normal combat options and slow them significantly, but offer greatly increased firepower in return. In addition to weapons, the game contains a new class of usable items called Equipment; these items are found in the game world and have various effects and functions, ranging from defensive screens to shield regeneration and flares. Only one piece of equipment can be carried at a time. The game's vehicular component has been expanded with new drivable and A.I.-only vehicles.
Halo 3 also adds new features not directly related to gameplay. One such feature, known as 'Forge', is a map-editing tool that enables players to insert and remove game objects, such as weapons and crates, into existing multiplayer maps. Almost all weapons, vehicles, and interactive objects can be placed and moved on maps with Forge. Players can enter Forge games and edit and manipulate objects in real time. A budget limits the amount of objects that can be placed. Another new feature are 'Saved films', which allows players to save up to 100 films of gameplay to their Xbox 360's hard drive, viewing the action from any angle and at different speeds. The Saved Films are only game data (not an actual video) and this allows the file sizes to be smaller than a true recording. All games are recreated in real-time on the Xbox 360 using the Halo 3 engine. Halo 3 offers a form of file sharing, where items such as saved films, screenshots, custom game modes, and Forge settings can all be uploaded to the 'File Share'. Anyone can browse user created content that has been uploaded to Bungie's website on a personal computer and tag it to automatically download to their console next time they sign into Xbox Live on Halo 3.