The probability formula of two events and events is p (a+b) = p (a)+p (b)-p (ab) (1).

With this formula, it is not difficult to deduce the probability formula of three events "and events" As long as (A+B) is regarded as a whole, it is obtained by (1).

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

=[P(A)+P(B)-P(AB)]+P(C)-P(AC+BC)

= P(A)+P(B)-P(AB)]+P(C)-[P(AC)+P(BC)-P(ABC)]

= P(A)+P(B)+P(C)-P(AB)-P(AC)-P(BC)+P(ABC)

Second, there is no better way to determine the probability that none of the three will happen, only by giving the answer. If you don't consider this problem, you can consider using the following formula: 1-P (at least one appears).

I hope I can help you!