The imread documentation states: The return value A is an array containing the image data. If the file contains a grayscale image, A is an M-by-N array. If the file contains a truecolor image, A is an M-by-N-by-3 array.
The image is likely not in color, but saved as grayscale or indexed. If an image is grayscale or indexed, then it has only size M*N, and does not have a page for each of R, G and B. Only truecolor images have size M*N*3. You can read more in the documentation:
To demonstrate this I created a small image (attached) and read it into MATLAB, where it is clear that it only has one page:
>> imread('test.bmp')
ans =
1 1 1 1 1 1
1 1 0 1 1 1
1 0 0 1 1 1
1 1 0 0 0 1
1 1 0 0 0 1
1 1 0 1 0 1
1 1 0 1 0 1
1 0 0 0 0 0
1 1 1 1 1 1
Depending on what kind of image it is you might need to get the map value from imread too.
