RowNum은 ORDER BY보다 먼저 실행된다.....

SELECT c1.* FROM (
    SELECT t1.CLASS_NO , t1.buyscore + t2.jjimscore total
    FROM (
        SELECT CLASS_NO, COUNT(*) buyCount, COUNT(*)*30 buyScore FROM CCLASS_MEMBER GROUP BY CLASS_NO) t1,
        (SELECT CLASS_NO, COUNT(*) jjimCount, COUNT(*)*10 jjimScore FROM FAVORITED_CLASS GROUP BY CLASS_NO) t2
    WHERE t1.CLASS_NO = t2.CLASS_NO
    ORDER BY total DESC
    ) t3 , CClass c1
WHERE t3.class_no = c1.class_no
AND rownum <= 3
;