I would recommend to export .obj file from blender, import it from threejs editor, apply all the material, export it using ‘export object’ from its file menu. And use
THREE.ObjectLoader to load the file in your application.
Regarding the transparency issue, I would recommend keeping the object ( Goalpost ) as a separate entity while exporting it from blender.
Change to object hierarchy in such a way that the transparent object, i.e the goal post, is at the very end of the object tree. Also use
material.side = THREE.DoubleSide.
Or else used use
.renderOrder property such that the goal post gets render only after all the other object get rendered for each rendering loop.