The Boston Licensing Board today approved plans by the McDonald's at 500 Geneva Ave. in Dorchester's Fields Corner to extend its current midnight closing time to 3 a.m. - but only for pick ups by third-party delivery services, so burger-mad night owls will have to punch in their orders at home. Nobody objected to the plan at a hearing yesterday. The franchisee at the location already operates similar wee-hour pickup service in Allston and at TD Garden.