I guess you have a Binary (Logic) mask you want to multiply the image by.
It is easy:
mMask = double(mMaskBinary);
for ii = 1:numChannels
mOutImage(:, :, ii) = mMask .* mInputImage(:, :, ii);
In the latest MATLAB releases (From R2016b) expansion is done implicitly.
Hence if you want to do a multiplication between $ M \times N $ image to $ M \times N \times 3 $ image you can just multiply them element by element and the expansion to the 3rd dimension will happen automatically like in the loop above.