Consider a non-empty sample space, Ω. Construct two non-empty subsets A, B ∈ Ω such that they are not mutually exclusive. Consider the probability measure function P that maps events in the power set of Ω to real numbers in the interval [0, 1]. Given P(A) = 0.6, P(B) = 0.7, and P(A ∩ B) = 0.3, verify Kolmogorov's third axiom by computing P(A ∪ B) and provide a detailed proof.