When Dolly Parton, Linda Ronstadt and Emmylou Harris began singing together in mutual admiration well before the release of the first Trio album in 1987, there was no Americana, no O Brother, no conventional category into which this music fit. The release of Trio: The Complete Trio Collection (out September 9 on Rhino) arrives in a wholly different context within which this music fits fine. Where Parton was (and is) a country icon, Ronstadt the queen of California country-rock and Harris the keeper of the legacy she shared with the late Gram Parsons, they very much sound like kindred spirits on a canon of material that refuses to settle into any genre. Trio was an unqualified smash upon release, among country and rock fans alike, with highlights extending from the songbooks of Phil Spector (“To Know Him Is To Love Him”) to Linda Thompson (“Telling Me Lies”) and Kate McGarrigle (“I’ve Had Enough”) to traditional spirituals (“Farther Along”). The belated followup Trio II didn’t arrive until 1999, patched together from busy schedules, and offered diminishing returns. The 20 tracks of alternate takes and material that didn’t make it onto either album (though some was later released on their solo albums) are sufficiently illuminating to help the listener appreciate anew just how pivotal these sessions were.