WITH t AS
(
SELECT '2000152001' chitemseq, '326' chitem1, '284' chitem2, '203' chitem3
UNION ALL SELECT '2000152002',       '40',       '70',       '20'
UNION ALL SELECT '2000152003', '5.39E+07', '1.85E+08', '1.17E+08'
UNION ALL SELECT '2000152003', '4.00E+06', '8.01E+05', '4.82E+05'
UNION ALL SELECT '2000152003', '6.95E+05', '8.01E+05', '4.82E+05'
)
SELECT chitemseq
     , chitem1, chitem2, chitem3
     , x
     , y
     , CASE chitemseq
       WHEN '2000152001' THEN CAST(CAST(x AS NUMERIC(12, 1)) AS VARCHAR)
       WHEN '2000152002' THEN CAST(CAST(x AS NUMERIC(12, 0)) AS VARCHAR)
       WHEN '2000152003' THEN CONCAT( CAST(x / POWER(10, y) AS NUMERIC(3,2))
                                    , 'E+'
                                    , RIGHT(CONCAT('0', y), 2)
                                    )
        END z
  FROM (SELECT chitemseq
             , chitem1, chitem2, chitem3
             , ( CAST(chitem1 AS FLOAT)
               + CAST(chitem2 AS FLOAT)
               + CAST(chitem3 AS FLOAT)
               ) / 3 AS x
             , FLOOR(LOG10(
               ( CAST(chitem1 AS FLOAT)
               + CAST(chitem2 AS FLOAT)
               + CAST(chitem3 AS FLOAT)
               ) / 3)) AS y
          FROM t
        ) a
;