You can use a loop, e.g.
acc = your 940x3 matrix
r = your 3x3x940 array
result = zeros(size(acc));
for k=1:size(acc,1)
result(k,:) = r(:,:,k) * acc(k,:)';
end
Depending on how r is defined (you didn't specify) you might need this instead:
result(k,:) = acc(k,:) * r(:,:,k);
Or you can explore using one of these FEX submissions (some of which require a C/C++ compiler to create mex routines):
