The arrival of the Prince of Cambridge means that the monarch, Queen Elizabeth, knows her three successors; Prince Charles, Prince William and the new baby.
This situation last arose in 1891 when Queen Victoria’s great-grandson, later Edward the Eighth, was born. The Queen’s son, King Edward the Seventh and her grand-son, King George the Fifth, succeeded to the throne…although in this case one other of her great-grandsons, later George the Sixth, became king in 1937 after Edward the Eighth (later Duke of Windsor) abdicated.