select salary 
from (select salary from tablename order by salary)
where rownum < 4;

Sorry ..above one is to find lowest threee...

This is for highest 3 salaries -

select salary 
from (select salary from tablename order by salary desc)
where rownum < 4;

In MYSQL:
select salary from (select salary from tbl_salary t order 
by salary desc) as u limit 3;

u is the table alias,which is a must.

for max 3 salarie:

SELECT TOP 3 sal from( Select DISTINCT max(sal)ORDER By sal
DESC) Order by sal;

select salary from(select salary from tablename order by 
salary desc) where rownum < 4