US stocks tumbled 3 percent on Wednesday, erasing gains for the week so far, as a spike in Italian bond yields fanned worries about contagion in the European debt crisis. In afternoon trade, the Dow Jones industrial average was down 426.70 points at 11,743.48. The Standard & Poor''s 500 Index was down 48.47 points at 1,227.45. The Nasdaq Composite Index was down 106.75 points at 2,620.74.