대략 이런 쿼리를 의도하시는 건지 ? 

SELECT 
	A.xkey, COUNT(colB) , COUNT(colC) , COUNT(colD)
FROM 
	A
LEFT JOIN (
    -- colB, colC, colD 는  B, C, D 구별용 임의 칼럼 
	SELECT xkey, 'B'  AS colB, NULL AS colC, NULL AS colD FROM B
	UNION ALL
	SELECT xkey, NULL AS colB, 'C'  AS colC, NULL AS colD FROM C
	UNION ALL
	SELECT xkey, NULL AS colB, NULL AS colC, 'D'  AS colD FROM D
) BCD ON A.xkey = BCD.xkey
-- WHERE A.특정조건
GROUP BY A.xkey
HAVING NOT ( COUNT(colB) = COUNT(colC) AND COUNT(colC) = COUNT(colD) AND COUNT(colB) = COUNT(colD) )