File under: triple threat. This Southern-inflected newcomer, its name shorthand for “Back Bay Beats,” scores on every front. There are creative takes on hoppin’ John, head-on shrimp and other Dixie staples, killer cocktails with housemade syrups and infusions, and excellent atmosphere: B3’s industrial-chic design incorporates high-quality sound and light systems that spotlight the prodigious talents of nightly live musicians booked via landlord Berklee College of Music.