HT Games
Single-Player Campaign
First-person shooters are all fairly similar, and Halo 3 is no exception. The first step to getting your skill level high is to play through the single-player campaign a few times. Start on normal difficulty and increase it each time you beat the game until you're playing on legendary difficulty.
Know the Maps
Learning all of the maps is crucial to succeeding at Halo 3. In single-player, starting on normal difficulty will give you plenty of time to explore the large maps and become familiar with them. In multiplayer, the maps are smaller and shouldn't take you long to memorize where the sniper spots and power-ups are hidden. The more familiar you are with a map, the more likely you are to have a good kill-to-death ratio, or KDR.
Stick With Your Preferred Weapons
If you are naturally good at sniping, keep your ammo full and keep your distance from enemies. In multiplayer, you will get rushed by melee or close-range players, so you will need to practice using escape routes and switching weapons.
If you prefer to run and gun, dual-wielding weapons such as the mauler work great for taking out enemies that come into your field of view.
Use Grenades Effectively
In single player, take the time to practice with grenades. Throw them around corners, over obstacles and into narrow spaces. Get a feel for how long it takes to throw them and how long before they explode. This can make a difference in multiplayer, because you can see an enemy approaching and can have a grenade waiting for them, ready to explode.
Practice
In Halo 3, especially when it comes to multiplayer, the best way to improve your skills is to practice. Practice the single-player campaign as well as the multiplayer modes frequently, and you'll get better. You'll remember more tricks for each level, know where weapons and armor are and easily go up the ranks.