The President or the Secretary concerned may drop from the rolls of the armed force concerned any Reserve—
((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.