Halo 3 Tips

Trigger Red
Whether you're trying to differentiate an enemy troop from an ally at a distance, or you're scanning the thickly armored legs of a Scarab, let your reticle guide you. If it turns red, let it rip.
High Ground
So often the fields of battle in Halo 3 take place on hillsides or near cliffs. Even indoors, you can often hop atop boxes, or climb a ramp or stairs to a second level. Gain the high ground whenever possible and you'll have a much easier time dispatching enemies and avoiding return fire.
Brutes First
Your primary focus in combat should be to pick off the most powerful enemies first. Usually, this means Brutes. Because Brutes tend to travel in packs, save your grenades and even rocket launcher fire for clearing out entire packs of these formidable opponents. The level design may also assist here. Look closely for exploding canisters near any tight-knit group of enemies.
Run 'em Over
Of course, a well-trained marine can dish out mayhem and death with the rear-mounted chain gun, but don't dismiss the devastation you can cause by simply running over your enemies. Entire camps of Covenant baddies can be cleared in a hurry, whether you're piloting a Warthog, Ghost, Brute Chopper, or even the slower moving Spectre.
Smack Em'… Unless
Melee strikes have always been a staple of the Halo games and Halo 3 is no exception. You can still find great success with one-hit kills when rifle-butting an enemy from behind. That said, you may want to rely a touch less on melee attacks as you do battle with the enormously powerful Brutes. There's more thunder in their fists than even Master Chief's, so watch out.
'Nade Tactics
Proper grenade use is always helpful, but on Hard or Legendary difficulty, it's critical.
Frag: Use these standard-issue grenades for long-distance throws. Unlike their plasma counterparts, these grenades roll after hitting the ground.
Plasma: The plasma grenade's greatest strength is its ability to stick to its target, so use these only when you're close enough for an accurate throw.
Spike: While the spike grenades don't travel as far, they do have a shorter fuse. Launch one at the feet of a group of enemies. They won't have time to flee outside the blast radius.
Brute T-Bone
The Brute Chopper shreds delightfully through enemy vehicles, but if you want to score a guaranteed kill, slam into the side of your enemy. Fender benders and head-on collisions may do damage, but a proper T-bone collision guarantees the most devastating results.
Riding Buddies
Whether it's the fast-traveling Mongoose, the more fearsome Warthog or a Brute Prowler, never ride alone if you can help it. Of course, this is most effective when playing co-op, but you may be surprised at the accuracy of the A.I.-controlled gunners. When entering a vehicle, always wait a second for a buddy to hop onboard.
Open Regeneration
The Halo 3 Regenerator offers Master Chief a quintessential Rambo moment, as you can safely drop the Regenerator (it constantly replenishes your shield) on the field of battle and unleash your wrath without fear of a swift death. This is a great tool to open up a large-scale battle, but make sure you're well within your weapon's effective range before employing this tactic.
Opposite Your Troops
Master Chief is worth an entire platoon of highly-skilled marines on his own, so don't clump up your forces by sticking too close to your allies. Instead, let your marines, and even the Arbiter and his Elites when they're with you, draw the attention of enemy forces. Then, slide along the opposite side of the battlefield to handle additional enemy forces, as well as to flank the foes with which your comrades are engaged.
Shield Hallways
The portable shield generator is nearly always useful, but it really comes into its own in indoor environments, specifically cramped hallways. Drop the shield between Master Chief and a cluster of enemies and then take your time picking them off one by one. Unlike other Halo 3-specific equipment, the portable shield doesn't operate on a timer. Once you've placed it, it's there forever.
Co-Op Compete
Co-op provides great opportunities for strategy. Don't just notch casual kills. Remember, your performance is graded against your friends, so get those grenade-sticks, headshots, multi-kills, and even pick off enemies your buddies have started on.
Keep a competitive mindset when playing co-op and you'll go a long way towards earning a respectable grade.
Co-Op Quadrants
Beyond competing with your friends, you should take full advantage of any cooperative play by breaking up the field of battle. So many confrontations in Halo 3 take place in sprawling "arenas," large open areas filled with enemies. Break down these arenas into separate quadrants, with each player responsible for a particular area of the environment.