Stocks tried for a comeback, but big selling in the last hour sealed a third straight loss Monday. The S&P 500 tumbled 1.6%, breaking its 200-day moving average, which it had been above since November 2012. It also breached support at the 1900 level. Energy, which makes up about 10% of the benchmark index, was one of the day’s worst-performing groups. The Nasdaq slumped 1.5%.