From an engineering perspective, either the butt plate return or the toe plates have to be separate pieces to allow for seasonal wood movement. The entire butt can not be encircled by all three parts and incorporated in to a 1-piece butt plate (even if you could figure out how to install it nice and tight). My vote is for the thickest piece of metal to be the one exposed to potential snagging when the butt is touching something (like your shoulder or the ground). That is, have the return and butt plate as one piece, and the toe plate as another, with the butt plate toe overlapping the toe plate, just like about 98+% of the guns out there are done.
Maple (seasonally) will move as much as 1/4" laterally (plain sawn) and 1/8" tangentially (quarter-sawn) over a 12" space. There is no discernible movement vertically. All wood projects with porous finishes (varnish, oil, etc.) need to allow for that (ingress / egress of airborne water vapor expansion / contraction) in their design and construction. In a perfect world we would all install our metal part inlets in the summer to make them that much tighter in the winter months.
The reason we need to elongate our barrel lug pin holes is that no piece of wood is going to have perfectly parallel grain to the barrel channel. The more curl there is (or waves in the grain), the more the wood will expand and contract perpendicularly to that grain direction.
I have a 48" barreled gun with a lot of curl in it, and in the summer the nose cap is .03" short of the muzzle. In February it's more like .10". If I hadn't elongated the wedge pin slots it might be impossible to extract the barrel at the extreme of the opposite season to the one I installed them in. It might have even cracked the stock. In fact, one of them WAS that way and I needed to wait a few months to extract the barrel and lengthen the slot.