Solution (source code)

= Solution

<Mutual independence> means that for every nonempty $I\subseteq\{1,2,3\}$,
$$
\boxed{\mathbb P\left(\bigcap_{i\in I}A_i\right)=\prod_{i\in I}\mathbb P(A_i).}
$$
Beyond the trivial single-event identities, this requires the three pairwise equalities and the triple equality $\mathbb P(A_1\cap A_2\cap A_3)=\mathbb P(A_1)\mathbb P(A_2)\mathbb P(A_3)$. <Pairwise independence> alone omits that last requirement.