((a)) No commissioned officer may be dismissed from any armed force except—

((1)) by sentence of a general court-martial;

((2)) in commutation of a sentence of a general court-martial; or

((3)) in time of war, by order of the President.

((b)) The President or the Secretary of Defense, or in the case of a commissioned officer of the Coast Guard, the Secretary of the department in which the Coast Guard is operating when it is not operating in the Navy, may drop from the rolls of any armed force any commissioned officer (1) who has been absent without authority for at least three months, (2) who may be separated under by reason of a sentence to confinement adjudged by a court-martial, or (3) who is sentenced to confinement in a Federal or State penitentiary or correctional institution after having been found guilty of an offense by a court other than a court-martial or other military court, and whose sentence has become final.