I am not sure whether your code is correct. You have an IF statement with a condition that will always be true i.e. IF new.login_col2= new.login_col2.
But I think the issue that you are facing here is that you are triggering after the record has been inserted. As a result, setting the NEW.login_col3 will not have any impact as the row is already in the database. Try doing the same thing before insert and this should work.