343 Industries has confirmed that Halo 3: ODST's campaign will be made available as part of Halo: The Master Chief Collection next month, along with the Relic multiplayer map remake.
On the Halo Waypoint blog, 343 Industries commented:
"Additionally, we’re excited about the progress of the Relic multiplayer map remake and the Campaign for Halo 3: ODST, both of which are on track to release within Halo: The Master Chief Collection next month with our next content update. Look for more details next month, including the final map name for the Relic remake, release date and more."
The game, originally released in 2009, will be available for free to anyone affected by The Master Chief Collection's severe online problems after launch. ODST took place before the events of 2007's Halo 3 in the city of New Mombasa. Many of the actors from Firefly were reunited for the game, which allowed players to take on the role of Orbital Drop Shock Troopers rather than Spartans.
When launched for The Master Chief Collection, the game will run at 1080p and 60fps. 343 Industries has also released a gallery of images from the remastered campaign which you can see below.