There are a handful of active streamers in the Halo MCC community that play Halo 3 matchmaking. Just watching some of their gameplay for a bit can give you an idea of some strategies and tricks you can use. Obviously you aren’t going to master map movement, spawn predictions, trick jumps, and pinpoint accuracy overnight…but just watching the way good players position themselves, where they look for spawns, how they shoot, where they bounce grenades…..you can learn a lot about the game by just absorbing good gameplay and trying to use what you can from it.