Halo 3 can't be shut down by 343, only Microsoft can do that. H3s servers are "watched" by a bot. And the only reason h2 was shut down wasn't because of the amount of people on it but the fact that Microsoft shut down all the Xbox original live games. Honestly, where do some of you get your information? Or are your posts merely assumptions?
Right now there are at least 20-40k people on every day, on a 5 year old game.
Versus reach with a population of about 80ish k people on a good day, and thats a 2 year old game.
Halo 3 is doing much better than reach in this perspective and read my other post concerning h3 servers being shut down