During a backstage break, the three Jonas brothers strip off their shirts for a clothing change. (Shriek!) As a camera moves in, heartthrob Joe, 19, bashfully tosses his T-shirt at the lens to block such an immodest view. (Double shriek!) Welcome to the wholesome puppy-love world of Jonas Brothers, the Disney-backed pop-rock act beloved by screaming adolescent girls worldwide. Concert, shot in 3-D mostly at two gigs last summer in Anaheim, Calif., reverentially showcases Joe, Kevin, 21, and Nick, 16, belting their hits, indulging in horseplay offstage, greeting ardent fans and generally coming off as swell guys. Fans will love it; adults sensitive to high-decibel squeals should drop the kids off and then go home and watch their old New Kids on the Block videos.