Given that a complex number \( z \) is represented in polar form as \( z = re^{i\theta} \), where \( r \) is the modulus and \( \theta \) is the argument of the complex number. Suppose \( z \) is a root of unity such that \( z^3 = 1 \). Calculate the probability that the product of three randomly selected such roots of unity is equal to 1. Let's think step by step and output the final answer within \boxed{}.
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