Supergroups are touch-and-go, hit-and-miss affairs. This one, though, is a definite hit, especially when gauged by the material co-written by members of the band rather than what’s brought in from other projects. Given that criterion, Tres Chicas–the trio of Whiskeytown’s Caitlin Cary, Hazeldine’s Tonya Lamm and Glory Fountain’s Lynn Blakey–is an outright wonder. “When Was the Last Time,” the only tune on the group’s Sweetwater debut penned by all three, is a beautiful killer, a slow burner begging a timid lover to respond, dressing mounting heartbreak up in gorgeous, three-way harmonies that sound like the strange, wondrous results of a lost session between Fleetwood Mac and Emmylou Harris. Though their schedules are packed, this group has pledged to play out more. We certainly hope so. Tickets are $5 and the show starts at 8 p.m.