For 2 sets: |A U B| = |A| + |B| - |A ∩ B|. For 3 sets: |A U B U C| = |A| + |B| + |C| - |A ∩ B| - |B ∩ C| - |A ∩ C| + |A ∩ B ∩ C|. The pattern alternates: add individual, subtract pairwise intersections, add triple intersection. This is the most commonly tested counting technique for sets.