It depends. Do you want to know the story or do you want to just play matchmaking online?
if you want to know the story, buy the first 3 games, or at the least look up the videos online. They have to have the videos from each game on youtube. This will let you know what is going on up to Halo 3. Halo ODST is not so much in line with the first 3 games story following Master Chief.
Now if you just want to play Matchmaking, forget halo 3, and go Get Reach tomorrow night at midnight. Reach is the new multiplayer of Halo, and 3 is not going to be really played anymore. Reach takes place before halo 1 so story wise it may be a good place to start but I'm sure the story will have references to older games in one way or another, as in things to come that you already experienced in halo 1.
Bungie was pretty brilliant in the story line of the game. Plus the forge on Reach, where you can make maps, is amazing. You can create any map you desire now as far as I understand.
In Short GET REACH!!!
