Hey Brian,

Yeung Chan has an excellent book out on jointery. It has some good 3 way
miter options.

It might be worth checking out.

I used one of the options in the book for a small carcass, the entire (all
6 sides) carcass was frame and panel. Frames were about 7/8" with 3 way
miters on all corners, using hidden floating tenons. If interested I'll
post a pic.

Cheers,

aw