I take B - C - D to mean (B - C ) - D, i.e. all elements of B - C not in D, or all elements in B not in C and not in D.
The given sets B and C are identical, so
B = {3, 6, 9, 12, 15, 18, 21}
C = {3, 6, 9, 12, 15, 18, 21}
===> B - C = { }
Then
(B - C ) - D = { } - D = { } (empty set)
