System Shock 3 is real, and in development at Otherside Entertainment. Little else is known save that Otherside is counting down to a reveal in the early hours of next Monday morning (14 December).
Otherside Entertainment was founded by Paul Neurath, who once worked at Looking Glass Studios and aided in the development of System Shock 2, which was released in 1999. The studio states on its own site that they are "a brand new game studio focused on rebooting classic game franchises."
The countdown can be found here, containing just the countdown timer and the "S S" of System Shock. The fact that it's System Shock 3 is found here, on another page under the same URL. The page prompts users to sign up for promotions and updates but offers no details on the project beyond its name.
Otherside is currently working on Underworld Ascendant, a new game in the fantasy RPG series Ultima Underworld. The project was launched on Kickstarter, successfully meeting its target ahead of a planned release by November 2016.
The page reveals that the Otherside team includes developers who worked on System Shock 2, Thief and modern games like Dishonored, The Last of Us and Bioshock Infinite – the last game from System Shock's director Ken Levine.
Kickstarter is a distinct possibility for System Shock 3, but we won't know for sure if that's what Otherside has planned until next week.