I have a *.3tz file. How can I load it with CesiumJS?
It is currently not directly possible to load 3TZ files with CesiumJS.
But you can simply unpack that 3TZ file. A 3TZ file is just a ZIP file, so you can just change the file extension to .zip and unpack the data into a directory, and load the data from that directory, using a local server.
(You could also use the 3D Tiles Tools convert function, but since this is only a ZIP, this is not even necessary here
(There also is a PackageServer.ts demo in the 3D Tiles Tools that can in theory serve the data directly from a 3TZ. But this is only a draft/demo - a proper implementation of this functionality is still in a pending/draft PR)