WITH t AS
(
SELECT 1 seq, 2000 val, 10 rep FROM dual
UNION ALL SELECT 2,  400, 10 FROM dual
UNION ALL SELECT 3, 3000, 10 FROM dual
UNION ALL SELECT 4,   20, 10 FROM dual
UNION ALL SELECT 5, 4000, 10 FROM dual
)
SELECT seq, val, rep
     , ROUND(DECODE(COUNT(*), 3, STDDEV(v)), 3) x
  FROM (SELECT seq, val, rep
             , LAG(val, 2) OVER(ORDER BY seq) v1
             , LAG(val, 1) OVER(ORDER BY seq) v2
             , rep v3
          FROM t
        )
 UNPIVOT (v FOR gb IN (v1, v2, v3))
 GROUP BY seq, val, rep
 ORDER BY seq
;