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.