A federal judge in San Francisco blocked a Reagan Administration plan to require random drug testing of 3 million interstate truck drivers nationwide. The Transportation Department had ordered testing to begin Wednesday for the truckers and more than 1 million other private employees in the rail, airline and maritime industries. U.S. District Judge Marilyn Hall Patel issued a temporary restraining order against random drug testing of truckers and interstate bus drivers. She allowed other testing programs for those employees, including pre-employment and post-accident testing, to take effect. The order expires Dec. 30, when Patel is to hear arguments on a longer-lasting injunction against all testing programs affecting truckers and interstate bus drivers.