If we have two sets, P and Q, then,
n(P U Q) = n(P) + (n(Q) – n (P ⋂ Q), where n denotes the number of elements.
For three sets, P, Q and R, the formula is given by;
n(PUQUR)=n(P)+n(Q)+n(R)–n(P⋂Q)–n(Q⋂R)–n(R⋂P)+n(P⋂Q⋂R)