addition rule for probabilities

CFA Level I
#1

So the rule is P(A or B) =P(A) + P(B) - P(AB)

I’m trying to figure out how this would work if:

You have three mutually exclusive items, all with probabilities of .5. Does that make P(A or B or C) = 1.5? Assuming it does, what does that mean? It can’t mean that there’s a 100% chance of at least one of A/B/C happening, or can it?

#2

You cannot have three mutually exclusive events, each with a probability of 0.5.

Remember that the sum of _ all _ mutually exclusive and collectively exhaustive events is 1.0, and probabilities cannot be negative.