What I'm suggesting is that you literally put the body on a table saw, or radial arm saw, and cut it into three pieces, right down the seams.
Then, you make two filler strips that are the width of the saw blade, and thickness of the body (to replace the material you removed by cutting it) and then glue the whole thing back together.
The problem you have is that wood has changed shape along the glue joints. It's no longer straight. The amount of force required to smush that joint back together will most likely damage the body by denting it. Even if you could force it back together, the glue joint will not hold for very long. The wood has changed it's shape.
You need to create new gluing surfaces that are straight and true. The only way to do this is to cut the body into pieces and re-glue it along the new, straight edges.
