Therea are two approaches, leading to the same solution

P(A or B or C) = P (A) + P(B) + P(C) - P(A and B) - P(A and C) - P(B and C) + P( A and B and C)

For proof, see:

https://math.stackexchange.com/questions/1291012/how-to-proof-formula-for-general-addition-rule-of-three-events

or obviously simpler, as most people here seem to have done : 1 - P(not A)*P(not B)*P(not C)