The officers of the state staff and detachment shall be selected and appointed by the adjutant general and commissioned by the governor if such appointees at the time of appointment shall have been, for at least three years immediately prior to appointment, a regularly commissioned officer of the National Guard; except that, where there is no qualified person in the National Guard available to fill a vacancy on the staff, an appointment may be made by the adjutant general if the individual so appointed qualifies for a commission pursuant to section 28-3-303.