select to_char(sysdate,'yyyymm') yearmonth from dual 
union
select to_char(add_months(sysdate,-1),'yyyymm') from dual 
union
select to_char(add_months(sysdate,-2),'yyyymm') from dual 
union
select to_char(add_months(sysdate,-3),'yyyymm') from dual

----------------------------------------------------------
       YEARMONTH
1    200712
2    200801
3    200802
4    200803

select *
from
(
select add_months(trunc(sysdate, 'month'), -rownum)
  from dual
 connect by rownum <= 3
)
/

select the_day, to_char(the_day, 'yyyymm')
from
(
select add_months(trunc(sysdate, 'month'), -rownum) the_day
  from dual
 connect by rownum <= 3
)
/