By John S. McClenahen For the third time in six months the Conference Board's Index of Leading Economic Indicators declined in July. Six of the 10 elements that make up the index fell, pulling the index down 0.4% to 111.7, reports the New York-based business research group. Economists generally expected an even bigger drop of 0.5%. Not surprisingly, falling stock prices were the greatest downer during July. Also pulling the index down were average weekly manufacturing hours, consumer expectations, the interest rate spread, vendor performance and building permits.