Supposing that there is only one record per employee per month, this one should work:
select e.*, a.Salary_Amount - d.Salary_Deposit as Salary_Pending
from
Employee_Detail e ,Salary_Allocation a, Salary_Deposit d
where
e.Emp_id = a.Emp_id
and
e.Emp_id = d.Emp_id
and
a.Salary_Month = d.Salary_Month
and
a.Salary_Amount - d.Salary_Deposit <> 0