Halo 3 Anniversary is not happening. This comes after a recent AMD event in South Korea where a Halo 3 image was shown, many assumed it as a leak that Halo 3 Anniversary is coming to PC, but devs repeatedly say no.

Source(s):

AMD VEGA teaser from South Korea https://www.youtube.com/watch?v=0L12B_YupK8

Frank O’Connor’s recent statement on Reddit – No Halo 3 Anniversary http://ift.tt/2oG18fG