Watch Supergirl Season 3 Episode 21 Online. Best and Free Online streaming for Supergirl TV Show. We picked the best sites to stream s03e21. Simply pick a site below and click "Watch Now!" button next to it. Some of the links may be broken, please upvote the working and good links so other users see those links for Supergirl Season 3 Episode 21 s03e21 at the top of the list.