Inclusion-Exclusion Principle on $3$ Sets

The video at https://www.youtube.com/watch?v=vVZwe3TCJT8 illustrates why the IE Principle on $3$ Sets actually works:

https://www.youtube.com/watch?v=vVZwe3TCJT8

These notes by Miriam Briskman are licensed under CC BY-NC 4.0 and based on sources.