Check the NDS special provisions for Wind and Seismic. They discuss a cantilevered diaphragm (3-sided box) on wood shearwalls.
It also states that you can not have a cantilevered wood diaphragm that is supported on masonry or concrete shearwalls for seismic (NDS 4.1.5) but there is no provisions for wind loads.
If you are going to keep the 3-sided box determine if the the diaphragm is flexible or rigid:
if it rigid, distribute the loads according to wall stiffnesses. Pretty much all the shear will go into the wall parallel to the applied load and the moment will be resolved into the walls perpendicular to the load.
If it is flexible you are out of luck.
The American Plywood Association has a free download for diaphragm design (L350) just go to there website.
