It was pretty much a given that there would be a new Call of Duty game this year, and since Black Ops developer Treyarch is up at bat right now (the series is currently on a three developer rotation, with each releasing a game every three years), Black Ops III was a pretty safe bet.
Now, the game is all but confirmed. Activision has released a "back in black" teaser trailer that shows very little, but it contains audio from previous Black Ops games and ends with a roman numeral three stylized the same way as Black Ops 2's "II."
It implies the full trailer -- potentially with gameplay -- will be released on April 26.
Prior to this trailer, Activision began teasing the game with a viral marketing campaign that uses Snapchat. They snuck a poster into a multiplayer map in Black Ops II with a Snapchat "Ghostface Chillah" (yes, that's the mascot's real name) image.
If you take a picture of that image with the Snapchat app you will start following the Snapchat user "CallOfDuty," and you'll start seeing updates from "Dr. Salim" in your Stories feed. The images have been incredibly cryptic, but it took no time at all for people to put two and two together and realize that Black Ops III is probably on the way.