The ranks of triple-threat musical theater writers—individuals responsible for book, music, and lyrics—are small. Michael John LaChiusa springs to mind, and Lionel Bart (Oliver!), Frank Loesser (The Most Happy Fella), and Sandy Wilson (The Boy Friend), but just try to think of others. Add to this exclusive club Francine Pellegrino, whose Molasses in January is premiering at the Theater Center. It’s an original book, based only on history—that of Boston’s molasses disaster of 1919, when a tank burst and sent syrup cascading through the streets, killing 21. Pellegrino is not overly experienced in any of these three skills, and she proves to be way better at one of them than the other two.