Here we go again. For the third straight day, markets tumbled after being spooked by the European debt crisis and national economy. In another roller-coaster day, the Dow Jones industrial average plummeted 100 points. Earlier in the day, stocks plunged, with the Dow dropping 308 points. The Standard & Poor’s 500 Index closed less than 1 percent lower, but at one point dropped 3 percent during the day.