Review the documentation for imread:
Specifically, the example with 'corn.tif' will show you different techiques for reading the seperate channels. A possible way to get what you want could be:
x = 1068;
y = 256;
z = 5;
stack = zeros(x,y,z);
for zz = 1:z
stack(:,:,zz) = imread('yourtif.tif', zz);
end
