There are many weapons in Halo 3, both different and the same from the previous two games.
Returning Weapons
Magnum: Same as in Halo 2, with a graphic update Rocket Launcher: Same as in Halo 2, it does retain its veichle lock-on. Sentiel Beam: It's color is now a more pure orange SMG Beam Rifle Sniper Rifle Carbine Fuel Rod Cannon Plasma Rifle Plasma Pistol Shotgun Battle Rifle Energy Sword Brute Shot Needler: No longer dual-wieldalbe, though is now shoots at a much faster rate and does a consideralby higher amount of damage.
New Weapons
Covenant Brute Mauler: a Heavy Duty large-barraled pistol used by Brutes. It can be dual-wielded. Brute Spiker: The Covenant equivlent to the UNSC SMG, a fast shooting dual-wieldable gun. Gravity Hammer: A large non-ranged weapon that can deflect large projectiles, such as rockets, grenades, and veichles, and do massive meele damage.
UNSC Assult Rifle: Returning from Halo: Combat Evolved, does slightly more damaged, but its' rounds are much smaller. Spartan Laser: A large, charge-up laser weapon. It must be fully charge in order to shoot. It has very small cartiages, but can destory entire veichles in one shot if aimed correctly.
Support weapons
Halo 3 introduces the idea of Support weapons, large weapons that require the player to be controlled third person. These weapons cannot be used meele, and the player cannot throw grenades while using these guns.
Missle Pod: A large Rocket-Launcher like gun with upgraded homing capabilities. Machine Gun: A very large machine gun weapon. Must be removed from a turret. Flame Thrower: A flame thrower that can torch opponents or set ground on fire.
Plasma Cannon: Similar to the machine gun, but shoots plasma rounds instead of bullets. Must be removed from a turret.
Grenades
Fragment Grenade: Slightly larger blast radius then previous games. Plasma Grenade: No longer solid blue, not as adhesive as in previous games. Spike Grenade: Looks like German grenades used in World War II, will stick to any surface. Firebomb Grenade: Does not do any damage, but creates a large flash that blinds all players within blast radius for several seconds, allow the person who through the grenade to get several shots in without his opponent knowing his location.