Plo with t two x axis

I am trying to plot two functions with two x axis with an additional one in the top ;

I followed the steps here http://www.mathworks.com/help/matlab/creating_plots/graph-with-multiple-x-axes-and-y-axes.html but it doesn't work ;

    a= [4.2058    4.1999    4.1250    3.9873    3.7788    3.5398    3.4203]
    b= [39.7875   39.7493   39.5321   38.9433   37.8052   36.1312   35.1034]
    c= [ 6     7     8     9    10    11    12 ]

what I want here is to set b as Y axis and a and c as x axis one in the top and one on the bottom with two diffrent xlabels

knowing that I have Matlab 2015a version