I recently published lib3mf crate provide a fully safe implementation for 3MF file format (https://3mf.io/), and while I was experimenting its integration on three-d-asset, found it worked so I am proposing its addition to three-d-asset as an optional dependency. Added a couple of sample files for testing from https://github.com/3MFConsortium/3mf-samples. Feel free to reject if you don't think it is convenient.

I was able to validate with a simple example applicaton on three-d, rendering one of the sample files included which is multi object, plus a cube:

Reference thumbnail from spec