What does it take for an indie band to get 30 million Spotify plays worldwide? An unusual group of three gifted singer-songwriters, with stunning harmonies that appeal to fans of modern folk-rock, but hearken back to the supergroups of the 60 and 70s. They’re widely hailed, from New Orleans to Hollywood. And now they’re subjects of a hot new film, “The Independents,” on the 2018 film festival circuit. We can’t really explain their magic—come see for yourself.