Yes, enclosing something in a 3R panel will give it a 3R rating. Actually, if you got a larger 3R panel, you could place the transfer switch and panel in it, and probably have a clean look. You may need to move to a NEMA 4 panel at those larger sizes though! Take a look at www.automationdirect.com,
they carry most of the hubbell panels and show prices...don't forget a back panel!