Ian Bostridge writes about a troika of singers that fired opera stages and composers’ imaginations in the early 1700s: Francesco Borosini, for whom Handel wrote two of his greatest operatic roles (Bajazet and Grimoaldo); Annibale Pio Fabri, another Handel regular and one of Vivaldi’s top stars; and John Beard, a London theater star who sang the heroes in many of Handel’s late oratorios.