I also wanted to quickly add different materials for floor/ceiling/walls.
At the beginning there was nothing to see with multi material.
In the docs:
BufferGeometryUtils
.mergeBufferGeometries ( geometries : Array ) : BufferGeometry
geometries – Array of BufferGeometry instances.
Merges a set of geometries into a single instance. All geometries must have compatible attributes. If merge does not succeed, the method returns null.
But in the source https://github.com/mrdoob/three.js/blob/master/examples/js/utils/BufferGeometryUtils.js :
mergeBufferGeometries: function ( geometries, useGroups ) { …
That’s what I used, but then the complete boxes are provided with the material one after the other.
So you would have to set the groups yourself.