((a)) A district court of three judges shall be convened when otherwise required by Act of Congress, or when an action is filed challenging the constitutionality of the apportionment of congressional districts or the apportionment of any statewide legislative body.

((b)) In any action required to be heard and determined by a district court of three judges under subsection (a) of this section, the composition and procedure of the court shall be as follows:

((1)) Upon the filing of a request for three judges, the judge to whom the request is presented shall, unless he determines that three judges are not required, immediately notify the chief judge of the circuit, who shall designate two other judges, at least one of whom shall be a circuit judge. The judges so designated, and the judge to whom the request was presented, shall serve as members of the court to hear and determine the action or proceeding.

((2)) If the action is against a State, or officer or agency thereof, at least five days’ notice of hearing of the action shall be given by registered or certified mail to the Governor and attorney general of the State.

((3)) A single judge may conduct all proceedings except the trial, and enter all orders permitted by the rules of civil procedure except as provided in this subsection. He may grant a temporary restraining order on a specific finding, based on evidence submitted, that specified irreparable damage will result if the order is not granted, which order, unless previously revoked by the district judge, shall remain in force only until the hearing and determination by the district court of three judges of an application for a preliminary injunction. A single judge shall not appoint a master, or order a reference, or hear and determine any application for a preliminary or permanent injunction or motion to vacate such an injunction, or enter judgment on the merits. Any action of a single judge may be reviewed by the full court at any time before final judgment.