Show that if A, B, and C are sets, then $\displaystyle \left|{A \cup B \cup C}\right|$=$\displaystyle \left|{A}\right|$ + $\displaystyle \left|{B}\right|$ + $\displaystyle \left|{C}\right|$ - $\displaystyle \left|{A \cap B}\right|$ - $\displaystyle \left|{A \cap C}\right|$ - $\displaystyle \left|{B \cap C}\right|$ + $\displaystyle \left|{A \cap B \cap C}\right|$.

So i know that $\displaystyle \left|{A \cup B}\right|$ = $\displaystyle \left|{A}\right|$ + $\displaystyle \left|{B}\right|$ - $\displaystyle \left|{A \cap B}\right|$. But i have no idea how to get to that final form