finding the total value in a table

In my report I have three formula fields namely 'A' 'B' 'C' and 'total' like shown in the below picture.

I have requirement like :

1) if A is null only B and C should be added in the total.

2) if B is null only A and C should be added in the total.

3) if C is null only A and B should be added in the total.

4) if A and B is null only C should be added in the total

5) if B and C is null only A should be added in the total.

6) if A and C is null only B should be added in the total.

these requirements should be present only in total formula!.

can anyone please tell me how to achieve this?