select code from gitxxxxxx
order by code;

 select * from (
select col1 from your_table_name
order by col1)
where rownum<4 

 
select col1 from (
select col1 ,row_number()over(order by col1)rnk
from your_table_name
)
where rnk<4 

 just change the order by to desc and accordingly change the where clause 

    SELECT ROWNUM,GPO.NO,GPO.GVN_CODE
      FROM GPOXXXXXX GPO,GPODTXXXX GPODT,GVNXXXXXX GVN
     WHERE GPODT.GPO_NO = GPO.NO
      AND GPO.GVN_CODE = GVN.CODE
      AND GPODT.GIT_CODE ='AA18'
 --   AND  ROWNUM < 4
    ORDER BY GPO.NO DESC

select rnum,no,gvn_code from(
select rnum,no,gvn_code,row_number()over(order by rnum desc)rnk
from test)
where rnk<4

 by the way,what version are you using ?
you can try this .

select rnum,no,gvn_code from (
select rnum,no,gvn_code from test order by rnum desc)
where rownum<4

select rnum,no,gvn_code from (
select rnum,no,gvn_code from test order by rnum desc)
where rownum<4