Watch the Supergirl season 3 finale promo! The CW has released the the promo for next week’s new episode of Supergirl, the season 3 finale! Check it out in the player below! Set to air Monday, June 18, the Supergirl season 3 finale is titled “Battles Lost and Won” and is offcially described as follows: “SHOCKING SEASON FINALE – Supergirl (Melissa Benoist) and team take on Serena in an epic battle for Earth.” Jesse Warn directed the episode which was written by Robert Rovner & Jessica Queller. “Make It Reign” has been directed by Armen V.