Believe it or not, just now plugged in my Halo 3 after all these years to try multiplayer with my daughter and, what, no multiplayer mode available? What am I missing? Did a few searches and a bit confused - I have just one disc in my case, and it’s not labeled “ODST” whatever that is. The available modes on my disc are: Campaign, Matchmaking, Custom, Forge, and Theater.
“Matchmaking” will be the option you are looking for out ofr those OP, that should allow you to play multiplayer 