Showtime has announced that Billions Season 3 will air in 2018.

The early numbers for Season 2 of Billions must be especially impressive, because Showtime has announced less than a month after the new season started that the series will come back for a third year in early 2018.
Showtime’s David Nevins said, “Billions continues to be a show on the rise, growing not only in audience, but in critical buzz and cultural relevance. It offers a unique mix of pure entertainment with sly commentary on our current have/have-not economy. Season 2 is off to a tremendous start, and has only whetted our audience’s appetite for what will come in Season 3.”
The show centers around the ongoing war between billionaire hedge fund operator Bobby “Axe” Axelrod (Damian Lewis) and U.S. Attorney Chuck Rhoades (Paul Giamatti) as the latter tries to nail Axelrod on his questionable tactics. Also starring on the series are Maggie Siff as Rhoades’ estranged wife -- and a former performance coach at Axelrod’s fund -- and Malin Akerman (Watchmen) as Axelrod’s fiercely protective spouse. They'll presumably be back as well for a third helping of politics, corruption and lots of money.
Source: Deadline