Halo 3: ODST is not a spin off its an expansion, hence the name Halo 3 in the beginning. Its like the Half-life episodes.
And now we see the problem with your argument. Any game that features the main protagonist and is considered cannon by the developer is NOT a spin off. Therefore if the next Halo game has the Master Chief fighting the forerunners in another galaxy it is not a spin off, it is a continuation of the main series. Hence why MGS:R, with Raiden who has played as a main character before, is not a spin off but a continuation of the series. Also note that a the next instalment in a series does not have to advance the time line yet it will still be considered as part of the main series.
Halo: Reach is the game that takes place just before Halo:CE. It involves the story of how all the Spartans except the Master Chief die and sets up the story for the first game. Hence it is not a spin off but considered part of the main series of Halo games.
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