extend operations above

The operations of union and intersection can be extended to more than two events.

For any three events $A$, $B$, and $C$, the event $A\cup B\cup C$ is the set of outcomes contained in at least one of the three events, whereas $A\cap B\cap C$ is the set of outcomes contained in all three events.

pairwise disjoint

Given events $A_1,A_2,A_3,\dots$, these events are said to be mutually exclusive (or pairwise disjoint) if no two events have any outcomes in common.