OMG stop. There is no Halo 3 Anniversary
Microsoft is not planning for Halo 3 Remaster edition. The news is first updated on reddit by Brian Jarrard in a single line comment saying "OMG stop. There is no Halo 3 Anniversary".
There won't be Halo 3 Remaster, but this doesn't indicate that this idea is completely dropped. Halo 3 came out in year 2007, and in order to celebrate 10th anniversary of Halo 3 there was a possible chance to get the remastered version which is not going to happen.
Before this Halo 2 got a 10 year anniversary treatment that came out with a fully re-mastered campaign. It allows users to select between Classic Mode and an enhanced graphic mode. Halo: The Master Chief Collection brings the entire Halo series with updated graphics including Halo 3. The collection offers re-mastered Halo 2: Anniversary, Halo: Combat Evolved Anniversary, Halo 3, Halo 4, Halo: Nightfall, and Halo 5: Guardians Multiplayer Beta.
Another update coming up under the Halo series is about Halo 6 which is confirmed not be coming in E3. There will be ample of new titles making their way out for the first preview along with much anticipated Xbox Scorpio, but Halo 6 won’t be part of it.