The CW has promised us that Legends of Tomorrow Season 3 will happen in 2017!

What might be the strangest assortment of superheroes ever put on TV will be back for another round next year. The CW confirmed Legends of Tomorrow Season 3 at the TCA Winter Tour.
While no other details are available, and it's far too early in the show's current season to make too many predictions, we can safely assume that there will be more new obscure heroes and villains joining the party in Legends of Tomorrow Season 3, and likely some new permanent faces on the Waverider.
The show has benefited this year from a less serialized format, focusing more on "cases of the week" that still serve a larger mystery. The result has been a series more comfortable in its own skin, allowing the terrific chemistry of the core cast to shine at nearly every turn.
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And if Legends of Tomorrow Season 3 were to do something really crazy like, I dunno, introduce the Legion of Super-Heroes, I wouldn't complain. After all, they did the Justice Society this year, so anything is possible, right? We'll update this with more information as we get it.