Gasoline prices have dropped below $3 a gallon at most U.S. gas stations, delivering a welcome lift to American consumers and retailers heading into the holidays. But the related oil-price drop has a thorny underside: It is threatening to slow the nation’s energy boom and hit the broader economy. The U.S. economy is still set to gain more than it loses from cheaper oil, at least at current prices. Lower prices at the pump give drivers more money to spend on discretionary items like restaurant meals and vacations, and they reduce costs for many businesses.