scatterhistogram() is its own class and does not provide access to its axes. Instead, use the older scatterhist(). It's output is a 1x3 vector of axis handles where the first handle is the axis to the scatter plot.
However, the marginal histograms are normalized and the boxplots will likely be at a much larger scale than the histograms. To see the axis scale, turn on the axes using,
I suggest putting the boxplots elsewhere, perhaps in their own axes or within the scatter axes.