- Melissa Benoist as Kara Danvers/Supergirl
- Mehcad Brooks as James Olsen
- Chyler Leigh as Alex Danvers
- Jeremy Jordan as Winn Schott
- Chris Wood as Mon-El
- Katie McGrath as Lena Luthor
- David Harewood as J'onn J'onzz/Martian Manhunter and Hank Henshaw/Cyborg Superman
- Odette Annable as Reign
Special guest cast
- Brenda Strong as Lillian Luthor/The Doctor
- Erica Durance as Alura Zor-El
- Adrian Pasdar as Morgan Edge
- Tamzin Merchant as Lyra Strayd
- Carl Lumbly as M'yrnn J'onzz
- Yael Grobglas as Gail Marsh/Psi
- Emma Tremblay as Ruby
Thirteen-year-old Kara escaped the doomed planet Krypton with her parents' help at the same time as the infant Kal-El. Protected and raised on Earth by her foster family, the Danvers, Kara grew up in the shadow of her foster sister, Alex, and learned to conceal the phenomenal powers she shares with her famous cousin in order to keep her identity a secret. Years later, Kara was living in National City and still concealing her powers, when a plane crash threatened Alex's life and Kara took to the sky to save her. Now, Kara balances her work as a reporter for CatCo Worldwide Media with her work for the Department of Extra-Normal Operations (D.E.O.), a super-secret government organization whose mission is to keep National City and the Earth safe from sinister threats. At the D.E.O., Kara works for J'onn J'onzz, the Martian Manhunter, and alongside her foster sister, Alex, and best friend, Winn Schott. Also in Kara's life are media mogul Cat Grant, James Olsen, a photo journalist who moonlights as Guardian, a masked vigilante, Lena Luthor, and Mon-El of Daxam, whose planet was ravaged by Krypton's destruction. As Kara struggles to navigate her relationships and her burgeoning life as a reporter, her heart soars as she takes to the skies as Supergirl to fight crime.
Confirmed plot points
- There will be a four-way crossover with Arrow, The Flash and DC's Legends of Tomorrow, with Supergirl playing a much more integral role than previously.
- The "New 52" Supergirl villain known as Reign has been confirmed to be the season's "Big Bad".
- Cadmus will continue its war against the DEO and Supergirl.
- The plot of the third season will revolve around what "means to be human", being something that will be addressed with all the characters.
- James Olsen/Guardian will have a new love interest, and will return to his roots as a journalist and photographer.
- Cat Grant will appear in multiples episodes in the season.
- She will appear in the season premiere.
- J'onn J'onzz is going to discover that he is not the last Martian on Mars and will introduce himself to his father played by Carl Lumbly, who also voiced J'onn J'onzz in the animated series of the Justice League.
- Lyra Strayd will return in this season and she will appear in more episodes than the last season.
- Kara Danvers will have new upgrades in her suit.
- There will be four new recurring characters in this season: Morgan Edge, M'yrnn J'onzz, Ruby and Psi.
- The season will pick up six months after the finale of Season 2.
- Mon-El will eventually make an appearance.
Confirmed new characters
- Morgan Edge: A "ruthless" real-estate developer with big plans for National City. Known for getting exactly what he wants - no matter the cost, nor the opposition - he'll almost immediately find himself at odds with Lena and Kara.
- Psi: A formidable opponent capable of using her enemies' minds against them. When she first meets Kara, the Girl of Steel will be affected in "surprising ways".
- M'yrnn J'onzz: The father of Martian Manhunter. Known for being a "pacifist religious leader" on his home planet of Mars, Papa J'onzz will apparently push his son in "unexpected ways".
- Ruby: An "independently minded" resident of National City whose obsession with Supergirl ends up putting her in grave danger.
- Oscar Rodas: Maggie Sawyer's father who is also a cop. 
|#||Episode name||Directed by||Story by||Teleplay by||Original airdate|
|1||"Girl of Steel"||TBA||Andrew Kreisberg||Robert Rovner & Caitlin Parrish||October 9, 2017|
|2||"Triggers"||TBA||TBA||TBA||October 16, 2017|
|3||"Far From the Tree"||TBA||TBA||TBA||October 23, 2017|
- ↑ 1.0 1.1 The CW renews seven series including 'Arrow,' 'The Flash,' 'Supernatural' and 'Crazy Ex-Girlfriend' - LA Times
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 SUPERGIRL SEASON 3 SYNOPSIS - Supergirl TV
- ↑ 3.0 3.1 3.2 ‘Supergirl’: Katie McGrath Promoted To Series Regular For Season 3 - Deadline
- ↑ Supergirl Gets Odette Annable For Season 3 Villain - Kryptonsite
- ↑ http://ew.com/tv/2017/05/26/supergirl-floriana-lima-season-3/?xid=entertainment-weekly_socialflow_twitter
- ↑ 6.0 6.1 6.2 6.3 Ask Ausiello: Spoilers on Supergirl, Scorpion, Lucifer, Younger, Will & Grace, New Girl, Bloodline, Arrow and More-TVLine
- ↑ ‘Supergirl’: Erica Durance To Take Over Alura Role From Laura Benanti
- ↑ A Flying Man From Heroes Is Coming To Supergirl - Kryptonsite
- ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 Supergirl Season 3 Adds Adrian Pasdar, Yael Grobglas and Alias' Carl Lumbly - TVLine
- ↑ Supergirl EP Talks True Four-Way DCTV Crossover, Nixes Doppelgänger Cameos - TVLine
- ↑ EXCLUSIVE ET Supergirl Interview
- ↑ Supergirl Casts 24 Vet Carlos Bernard as Maggie's (Spoiler) in Season 3 - TVLine
- ↑ 13.0 13.1 13.2 1ST THREE EPISODE TITLES S3 REVEALED - Supergirl.Tv