For filtering, it is probably not a good idea to convolve with d or D. Instead, you should filter separably. For example, in 2D, you could do,
f = conv2(a,b,M);
For element-wise multiplication in 3D, I similarly don't think building D is the way to go, as it creates an unnecessary extra array. I would probably do,
f=bsxfun(@times,V, a(:)*b(:).');
f=bsxfun(@times,f, reshape(c,1,1,[]));
But, if you really insist on building D, you could do
D = bsxfun(@times, a(:)*b(:).', reshape(c,1,1,[]));
Or, you could do
d=a(:)*b(:).';
D=reshape(d(:)*c(:).', size(V));
