NORRISTOWN, Pa. — Iconic entertainer Bill Cosby was convicted Thursday on three counts of sexual assault, a decision that punctuates one of the most thundering falls from grace in American cultural history.Once one of the nation’s most admired men, a pioneering African-American actor beloved for his role as Dr. Cliff Huxtable on the 1980s megahit “The Cosby Show,” Cosby was recast in a suburban Philadelphia courtroom as a merciless predator and sexual deviant.