This is really confusing to me... how could I prove this?

If A1, A2, and A3 are three events and the union of A1 and A2 != 0 and the union of A1 and A3 !=0 but the union of A2 and A3 = 0 show that

P(at least one Ai) = P(A1) + P(A2) + P(A3) - 2P(A1 n A2)