그룹핑 기준이 뭔가요?
1. item
2. item, color

WITH t AS
(
SELECT 'A' item, 'red' color, 100 price
UNION ALL SELECT 'A', 'red' , 150
UNION ALL SELECT 'B', 'blue', 200
)
, copy_t AS
(
SELECT 1 lv
 UNION ALL
SELECT lv + 1
  FROM copy_t
 WHERE lv <= 99
)
SELECT a.item
     , c.color
     , c.price
  FROM (SELECT item
             , CEILING(COUNT(*) / 3.) * 3 cnt
          FROM t
         WHERE item IN ('A', 'B')
         GROUP BY item
        ) a
 INNER JOIN copy_t b
    ON b.lv <= a.cnt
  LEFT OUTER JOIN
       (SELECT item, color, price
             , ROW_NUMBER() OVER(PARTITION BY item ORDER BY item) rn
          FROM t
        ) c
    ON a.item = c.item
   AND b.lv   = c.rn
 ORDER BY a.item, b.lv
;

-- 원본 테이블 명을 data_table 로 정의 했습니다.
with t_null as (
  select 1 as num from dual union all
  select 2 as num from dual  
)
,tmp as (
  select item, 3 - mod(count(1),3) as num
  from data_table
  group by item
)
select t.*
from (
  select * from data_table
  union all
  select t1.item, null, null
  from tmp t1
  ,t_null t2 
  where t1.num >= t2.num
  ) t
where item in ('A','B')
order by item, color

흥미있는 내용이여서 저도 따로 쿼리를 작성해 봤습니다.

(해당 데이터 row에 null row 를 추가하는 형태 입니다)