Histogram implies that you are binning the data, and the coordinates are associated with the edge or central value of the bin. 3D histogram implies that you are binning the data with respect to two different coordinates.
However, in the diagrams you show, at least to the resolution you present, there is no binning of data. We can see that one of the independent axes is length 600 and the other is length 800, but uint8 has at most 256 different values, and int16 or uint16 there is no obvious reason why you would bin in widths of 41 input values in one direction and in widths of 55 input values in the other direction.
It does not seem likely that you want histograms at all. I would suggest that you are instead looking for bar charts.
a = imread('flamingos.jpg');
h = bar3(a(:,:,1)); set(h, 'EdgeColor', 'none'); title('red')