Producers of upcoming Broadway musical “Motown” are going the route of online open auditions to find a young thesp who will play more than one significant role in the brewing tuner.Moppet will be tasked with portraying no fewer than three Motown greats: a young version of Michael Jackson as well as youthful incarnations of Stevie Wonder and Berry Gordy, who is one of the producers of the show. “Motown” will be the second musical this season to launch a casting search via the Internet: The fall revival of “Annie” also engaged in a similar hunt, solicitiing video auditions from young thesps. The tactic has a couple of advantages, not only exposing casting directors to new talent but also establishing a public profile for the production long before rehearsals have begun. (The casting of the title character for “Annie,” to be played by Lilla Crawford, was announced on the “Today” show on NBC.) For the online audition, “Motown” producers are asking kids between 8 and 11 to perform one of three songs — “I Want You Back,” “I’ll Be There” and “Who’s Loving You,” all performed by the Jackson 5 — in a video to be uploaded to YouTube for consideration. Casting is part of initial phases of production on the new tuner, slated for a spring 2013 bow at a Nederlander theater still to be determined. Charles Randolph-Wright helms the show, with book by Gordy and music drawn from Motown’s deep catalog of familiar tunes from acts including Smokey Robinson and the Miracles, Diana Ross and the Supremes, Marvin Gaye, the Temptations, Wonder and Jackson. Storyline of the tuner follows the establishment and development of Motown Records, founded by Gordy in 1959, and its stable of stars. Kevin McCollum (“Avenue Q,” “Rent”) and Doug Morris, topper of Sony Music Entertainment, produce with Gordy.