Cardinality of a union (3 sets).
For any finite sets \(A\text{,}\) \(B\text{,}\) and \(C\text{,}\)
\begin{equation*}
\card{A \cup B \cup C} = \card{A} + \card{B} + \card{C} - \card{A \cap B} - \card{A \cap C} - \card{B \cap C} + \card{A \cap B \cap C}\text{.}
\end{equation*}