U = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 }
A = { 3, 6, 9, 12 }
B = { 1, 3, 5, 7, 9, 11 }
C = { 4, 5, 6, 7, 8, 9 }

What is the answer of
(A ∩ B ∩ C)=