One box, copied three times, each copy a different colour. It shows how a material node picks which objects it applies to, using the object index as its group, and how it points at a material by relative path.
mat/MeshBasic geo/Material
Examples
Every one of these opens in the browser editor, running, with its node graph laid out for you to take apart. Nothing to install and no account needed.
A grid of spheres shaded by a material built node by node. Roughness and metallic are read from point attributes, so the surface changes across the field instead of being one uniform value.
This example creates a series of objects in a line. If you observe them from either ends of the line, you will see the objects behind the first one are tinted by the ones before them. Their materials' alpha is what makes them see-through. The last one also goes through a geo/ObjectProperties, which sets a transparency mode on that object alone. It is left on AUTO, so the material still decides and the sphere renders like the others. Set it to OPAQUE and that one sphere draws solid, with no change to its material.
geo/ObjectProperties mat/MeshBasic
A two-colour nameplate written to a single .glb, the interchange file Blender, three.js and the game engines read: every object keeps its own transform and its material's colour, and no preparation transform is needed because glTF shares Polygon's Y-up, meter conventions.