The CW is wasting no time with a renewal this year. Supergirl Season 3 will happen in the fall!

Well, this is a nice change of pace. When Supergirl's first season aired on CBS, the network hemmed and hawed until the eleventh hour about whether or not the show would see a renewal. Well, not this time. With the show moving to the CW for the second season, the network wasted no time this year in announcing that yes, SupergirlSeason 3 will happen.
While obviously we've still got a little ways to go before season two even finishes, the producers are already starting to put elements of Supergirl Season 3 in place. Katie McGrath will be a series regular when things get going next year, according to Deadline.
The series got off to a strong start by introducing Tyler Hoechlin as Superman alongside Ms. McGrath's Lena Luthor, and then promptly forged its own path, introducing more alien characters and taking more chances with its stories.
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We're glad to have confirmation on this, and this is the article that we'll continue to update as more information becomes available.
Supergirl Season 3 Release Date
Expect the new season to kick off in early October. We'll update this with the official date when it becomes available.