The surest thing at the coming Olympic Games in London — more than the swimmer Michael Phelps, or the sprinter Usain Bolt, or even the American men’s basketball team — may be a 23-year-old Japanese gymnast nicknamed Superman. Four years ago, at the Beijing Games, Kohei Uchimura finished second in the men’s all-around competition, based on his performance in floor exercise, pommel horse, rings, vault, parallel bars and horizontal bar. Since then Uchimura has won three consecutive world championships in the event, something no other male gymnast has done. And he didn’t just edge his way to those golds by hundredths or tenths of a point; he won by overwhelming, multiple-point margins. (In 2009 and 2011, he finished first in four of the six disciplines.) “After he competes in London,” says Tim Daggett, an Olympic gold medalist in gymnastics and a commentator for NBC, “I think he’ll have enough titles . . . to say he’s the greatest that ever lived.”