Suppose that an experiment has four possible outcomes: A, B, C, and D. If P(A) = P(B) = P(C) and what is the probability of each event?

Answer 1

The probability of D is
P(D) = 1 - 3 P(A)

You have already stated the other probabilities. Specific numerical values depend upon what P(A) is. There is a range of possibilites from 0 to 1/3.