Use the two different situations when the probability of \( A \cup B \) has common or no common elements, and extend in finding the probability of \( A \cup B \cup C \).
I. Use Venn diagram to show the intersections of two or more sets, to solve problems applying the probability of \( A \cup B \), or apply the operation rule:
\[
\begin{array}{l}
P(A \cup B)=P(A)+P(B)-P(A \cap B) \text { or } \\
P(A \cup B \cup C)=P(A)+P(B)+P(C)-P(A \cap B)-P(A \cap C)-P(B \cap C)+P(A \cap B \cap C)
\end{array}
\]