When fabricating a mirror frame, we do a deeper inside edge rabbet depression, usually an extra 3/8″ deeper than a plain mirror frame sequence.  Then we simply add 1/8″ strips of alternating species to create 3 stripes if you will.  It is usually done before we miter or assemble the frame but can be done afterwards too. 

The end result once the mirror is installed against the strips is that it appears to be 6 strips deep, a very simple step to add a subtle enhancement to the visual end product.