--シートが3番以降で3列空席（3列の和が0）のシートにフラグをつける
WITH TARGET_SEAT AS
(SELECT *,
CASE WHEN SEAT_NO > 2 AND
SUM(RSV_STATUS)
OVER(PARTITION BY LINE_ID
ORDER BY SEAT_NO
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) = 0 THEN 1
ELSE NULL END AS FLAG
FROM SEAT_RESERVE)

--先頭番号表示＋まとめ
SELECT 
LINE_ID AS LINE,
(SEAT_NO - 2) AS SEAT_F,
SEAT_NO AS SEAT_T
FROM TARGET_SEAT
WHERE FLAG = 1
ORDER BY LINE_ID DESC,SEAT_NO;