If you mean 3 rankings in the system of the issued certification, it would be '최우수, 우수, 양호'.
But if you mean the qualifications of the applicants, it would be '우수, 양호, 미흡'. And the person with '미흡' could not even apply because of the lack of the needed qualification(score, education, career etc).
For 4-ranking system, '우수, 양호, 보통, 미흡',
though sometimes I find it as '최우수, 우수, 보통, 미흡'.