How can I apply the inclusion exclusion principle when I have $$|Q|+|P|+|R|-|Q\cup P| -|Q\cup R|-|P\cup R|+|Q\cap P\cap R|$$
I'm not sure how to convert the unions into intersections here $$"|Q\cup P| -|Q\cup R|-|P\cup R|"$$ so that I can apply the inclusion exclusion principle.