The easiest way to work this out would be to:
SELECT * FROM `employee_table` ORDER BY `sal` DESC LIMIT 1 OFFSET 2;
Which orders employees by salary and then tells db to return one result (1 in LIMIT) counting from third row in result set (2 in OFFSET). It may be OFFSET 3 if your DB counts result rows from 1 and not from 0.
This should work in MySQL and PostgreSQL.