The MATLAB documentation that introduces multidimensional arrays names the third dimension as "page", just as the first and second are named "row" and "column" respectively. Quoting from that linked page (take a look!):
The first references array dimension 1, the row.
The second references dimension 2, the column.
The third references dimension 3. This illustration uses the concept of a page to represent dimensions 3 and higher
Note that there can be more than three dimensions! Also note that in mathematics a matrix refers exclusively to a 2D array, if it has more than 2D then it is an array. This means actually q from the original question is not a matrix, as it has at least three pages: q(:,:,3).
EDIT: If you are working with images, then note that an RGB image stores each of the R, G, and B colors on separate pages, in this order:
data = imread(filename);
data(:,:,1) % red
data(:,:,2) % green
data(:,:,3) % blue
