Principal of Inclusion-Exclusion

First, Cardinality:

The amount of items in a set.

Principal of Inclusion-Exclusion:

Also: $|A\cup B|=|A|+|B|-|A\cap B|$ → Cos you’d otherwise be counting their overlap twice instead of once.
Note: $|A\cap B|$ could be $\varnothing$ if A and B are mutually disjoint.
What about for sets $A$, $B$, $C$ ?
Same idea: $|A\cup B\cup C|=|A|+|B|+|C|-|A\cap B|-|A\cap C|-|B\cap C|+|A\cap B\cap C|$