Alongside Melissa Benoist, who stars in the title role, principal cast members Mehcad Brooks, Chyler Leigh, Jeremy Jordan, Chris Wood and David Harewood return from the second season.

They are joined by Katie McGrath, who was promoted to a series regular from her recurring status in the previous season, and new cast member Odette Annable.

[1] On the parallel world of "Earth-X", a Nazi regime rules the planet where an archer, Dark Arrow, is the Führer, and personally hunts the resistance forces.

Alex meets the Legends' leader Sara Lance / White Canary, taking the first step to recover from her break up with Maggie.

The wedding ceremony is interrupted by Earth-X Nazi invaders, led by Dark Arrow, his Kryptonian-powered wife Overgirl, and his partner Prometheus.

Brainiac 5 decides to stay on present-day Earth because a distant relative has started wiping out all other AI and asks Winn to take his place with the Legion.

[62] The season also sees Kara Danvers / Supergirl and Mon-El again parting ways; Queller compared this separation to that of Humphrey Bogart and Ingrid Bergman's characters in the ending of Casablanca (1942), saying, "They both have destinies in different times as heroes.

"[63] Main cast members Melissa Benoist, Mehcad Brooks, Chyler Leigh, Jeremy Jordan, Chris Wood and David Harewood return from previous seasons in their respective roles as Kara, James Olsen, Alex, Winn Schott, Mon-El and J'onn J'onzz / Martian Manhunter.

[68] In May 2017, The CW president Mark Pedowitz officially announced plans for a four-show Arrowverse crossover event, crossing over episodes of the television series Supergirl, The Flash, Legends of Tomorrow, and Arrow.

The site's consensus reads, "Heavier themes lead to higher stakes, but Supergirl gives its eponymous heroine and her fellow supers plenty of room for growth, creating a well-balanced, engaging third season.

"[98] Jesse Schedeen of IGN rated the season premiere 7.5 out of 10, saying it "largely focusing on what's worked best for the show in the past – emphasizing Kara Danvers' never-ending struggle to balance her personal and superhero lives and the inspirational symbol Supergirl has become to National City", but added that "the lackluster debut of new villain Morgan Edge does little to suggest the series will improve on its perpetual villain woes".