Notę: There are intersecting parts here. W3 intersects with W1 and W2. This was how the 3d model was when i came across it and I forgot to fix it. It's easy enough to fix though.
Build W3 as normal. then you use it as a template for cutting out a section of W1 and W2 before glueing W2 and W3 together. Use this picture as a reference.