LYNN — The most popular menu item at PapiVivi, a new restaurant on Boston Street, is Vivi’s Tripleta, a three-meat Puerto Rican street sandwich with roasted pork shoulder, ground beef, and chicken breast marinated in sofrito, pressed in fresh bread with and topped with mayonnaise, ketchup, garlic, cilantro, potato sticks, and a signature PapiVivi sauce.…