You can only use one account to play online in Halo 3, all you do is sign in a second, third, and even fourth controller as a guest. This way you can play with up to 4 people at a time on one screen with only one xbox live account. There are limitations however, you will not be able to play in an "ranked" playlists, but you are allowed to play in the "social" playlists.
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Also, your guests will not be able to use a microphone to talk, but the main account can still use his.
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