This is how you get smooth shading on your meshes from Blender -> GLTF -> THREE (with a MeshPhysicalMaterial)
1. Export your mesh with smooth shading toggled on in the object’s Tools -> Edit menu.
2. Make sure
material.flatShading=false is set on your material.
3. If you still have problems, see this https://github.com/mrdoob/three.js/issues/347