If you want to play co-op or firefight with 2 friends on split-screen and use your own profile pay attention. You can use your own gamer profile by clicking the A button to start the game; Then in the 5 seconds it gives, sign in using your gamer profile. After the game starts a message will appear saying that 3 players cannot play, and will start you off with just the 2 first players. Quit the game, then start the game over. Right or a little bit after the 5 seconds is up click A, it might ask you to choose a memory device, so just press A quickly. It might not let the 3rd player spawn on the map for firefight, if this happens just restart the game, do not quit. Also, for firefight try to let the 3rd player select his/her loadout first. There will no moments where the game freezes and all players will gain experince. If the 3rd player does not spawn with a certain weapon or falls out of the falcon on the first level you can restart. You will have to do this after every game if campaign scoring is on.