I cannot find documentation for several materials - I know that MultiMaterial is out of date, but how does it work now?
Does three.js support this, and if so, how do faceVertexUvs map to multiple materials (I mean a practical example)
To apply multiple materials to an object, you can now assign them as an array for a material property.
mesh.material = [material1, material2, material3];
See the Migration Guide (r84 → r85) :
MultiMaterial deleted. Use an array instead.
MultiMaterial
faceVertexUvs is a multidimensional array where the first index is the index of the material.
faceVertexUvs
geometry.faceVertexUvs[ materialIndex ][ faceIndex ][ vertexIndex ]
(Link: three.js - geometry.faceVertexUvs [0] [0] [index] is not the same as geometry.vertices [index] )
Three.js docs may be a little better in both cases.