Hi @Alan_Rose,
The tillingScheme property of CesiumTerrainProvider is read-only, and not part of the constructor options. CesiumTerrainProvider automatically sets the value of tilingScheme based on the layer.json file that it receives from the server.
Both Cesium World Terrain and Maptiler’s quantized mesh tileset are provided in EPSG:4326. If you need tiles in EPSG:3857, you will either need to find a different data source, or re-tile the terrain data yourself.
Can I ask why EPSG:3857 is necessary? As discussed in this older thread, you can combine 4326 terrain with 3857 imagery in CesiumJS. Are you wanting to use the terrain tiles in a different viewer?