The price of diesel fuel in the United States declined for the third straight week, dipping 2 cents to $4.01 per gallon, the lowest level in almost three months. Meanwhile, increased demand for gasoline for vehicles and to fuel generators brought long lines to filling stations in the ravaged portions of New Jersey and New York City as residents struggle to recover from Hurricane Sandy in the prior week. But diesel lines were short, trucking industry leaders said. Diesel’s price decrease followed an 8.6-cent drop the week before. Diesel is now at its lowest price since the $3.965-a-gallon average price on Aug. 13. Diesel has fallen in five of the past seven weeks. The average gasoline price fell 7.6 cents, to $3.492 a gallon, after an 11.9-cent drop the previous week. It was the fourth straight decline for gas, which has fallen in six of the past seven weeks.