Personally I would use an excel sheet.You can keep it updated.
a) You can have it auto sum
b) and auto average for you.
c) You could even write a formula letting the students know what mark they needed:
a) =SUM(A3:C3)
Explanation
A3:C3 = the cell range (increase as required)
b) =SUM(A3:C3) / 3
Explanation
A3:C3 = the cell range (increase as required)
3 = the number of assignments (increase as required)
c) =(4-(SUM(A3:C3) / 3)) *3
Explanation:
4 = the desired average (increase as required)
A3:C3 = the cell range (increase as required)
3 = the number of assignments (increase as required)
Tip: This will give you the total for all assignments.
For the last assignment it will show it exactly the mark required.
Hope that helps.