The broadest measure of the U.S. trade gap shrank to a three-year low of $30.9 billion in the third quarter as investment earnings jumped back into the black and the merchandise trade deficit narrowed, the Commerce Department said Tuesday. The $30.9 billion current account deficit was the smallest since the $27.6 billion mark in the third quarter of 1985, the department's Bureau of Economic Analysis reported. The current account is the most complete trade statistic because it includes both merchandise trade and services, primarily the flow of investments between the United States and its trading partners.