That depends on what you mean by plotting a matrix. I recommend looking at this documentation page for a picture that looks like what you want. Click on the word immediately above a picture to visit the documentation page for the function used to create that picture. [I suspect you probably want surf or maybe scatter3.]
That page does not list functions in products other than MATLAB, so if you have some toolboxes available and you want to see what functions those toolboxes provide to create specialized types of graphics, follow the instructions on this documentation page to see what other functions are available in the Plots tab in the Toolstrip.
