Start out by making sure that you have at least 3 controllers with gamertags or 2 other friends that would like to get easy experience points with you. Have each person log into their Xbox Live gamertag and start playing Halo 3. Open up a multiplayer game and change the gametype to Social Matchmaking on Rumble Pit.
Rumble Pit is a social free for all gametype in Halo 3 that allows up to 5 players to be in a single game. First, second and third places will all gain experience points in Rumble Pit on Halo 3. Have all 3 players in the same pregame lobby and start the game.
No matter what gametype you get, make sure that none of the players get any kills or any points of any kind. Allow the game to play out with the other 2 players winning. Once the game is complete, all 3 of your friends or other gamertags on your Xbox 360 will tie for third place. Since third place still gains experience points in Rumble Pit, all 3 players will gain experience points.
You do not even need to sit at your Xbox and watch. You will just gain experience points for sitting in the game and doing absolutely nothing. You can repeat this process as many times as you want, just make sure that none of the players gain any points because then they will be ahead of the other 2 and neither of them will gain any experience points after the game is over.