Here is my statement for Oracle 8.1.5 and 8.1.6.
But Oracle 8.1.7 introduced new <b>analytical</b> functions
which are designed for this kind of problems .
 
drop table testtab;
create table testtab (
   x     number(3),
   n     number(10)
);
 
insert into testtab values ( 1, 3 );
insert into testtab values ( 1, 30 );
insert into testtab values ( 1, 8 );
insert into testtab values ( 1, -8 );
insert into testtab values ( 1, 12 );
insert into testtab values ( 1, 15 );
 
insert into testtab values ( 2, 3 );
insert into testtab values ( 2, 14 );
insert into testtab values ( 2, 5 );
insert into testtab values ( 2, 18 );
insert into testtab values ( 2, 12 );
insert into testtab values ( 2, 15 );
 
insert into testtab values ( 3, 5 );
insert into testtab values ( 3, 0 );
 
insert into testtab values ( 4, 5 );
insert into testtab values ( 4, 0 );
insert into testtab values ( 4, 4 );
insert into testtab values ( 4, 5 );
 
-- all values
select * from testtab
   order by 1,2 desc;
   
         X          N
---------- ----------
         1         30
         1         15
         1         12
         1          8
         1          3
         1         -8
         2         18
         2         15
         2         14
         2         12
         2          5
         2          3
         3          5
         3          0
         4          5
         4          5
         4          4
         4          0
 

-- my solution                 
select x, n from (
   select x, n, rn from (
      select x, n, rownum rn from (
         select x, n from testtab
            order by x, n desc
      ) 
   ) a 
   where a.rn = 
      ( 
         select min(rn) from (
            select x, n, rownum rn from (
               select x, n from testtab
                  order by x, n desc
                  ) 
            ) b
            where b.x = a.x
      )  
   union all
   select x, n, rn from (
      select x, n, rownum rn from (
         select x, n from testtab
            order by x, n desc
      ) 
   ) a 
   where a.rn - 1 = 
      ( 
         select min(rn) from (
            select x, n, rownum rn from (
               select x, n from testtab
                  order by x, n desc
                  ) 
            ) b
            where b.x = a.x
      )  
   union all
   select x, n, rn from (
      select x, n, rownum rn from (
         select x, n from testtab
            order by x, n desc
      ) 
   ) a 
   where a.rn - 2 = 
      ( 
         select min(rn) from (
            select x, n, rownum rn from (
               select x, n from testtab
                  order by x, n desc
                  ) 
            ) b
            where b.x = a.x
      )  
) 
order by 1, 2 desc;
 

         X          N
---------- ----------
         1         30
         1         15
         1         12
         2         18
         2         15
         2         14
         3          5
         3          0
         4          5
         4          5
         4          4