Welcome to the community 
From my understanding, it is not possible to download 3D Tiles assets from Cesium ion. For some users, an on-premise solution, which allows them to tile data locally, may be appropriate. You can learn more about that here:
If you would like more information on downloading data from Cesium ion, here is a post where a user asks a similar question:
Let me know if you have any other questions or concerns!
-Sam