Exporting 3MF from Unity is not supported yet :(

Unity is a game engine made by Unity Technologies. It supports more than 25 different platforms. This engine is used mostly to design three-dimensional, two-dimensional, virtual reality, and augmented reality games. It has been adopted not only by video gaming industry, but also by film, automotive, architecture, engineering and construction.

Unity enables users to develop games and experiences both in 2D and 3D, and offers a primary scripting API in C#, as well as drag and drop functionality. Within 2D games, Unity provides importation of sprites and a 2D world renderer. For 3D games, it allows specification of texture compression, mipmaps, and resolution settings for each platform it supports. Also, it provides support for bump mapping, reflection mapping, parallax mapping, screen space ambient occlusion, dynamic shadowing, render-to-texture and full-screen post-processing effects.

The 3D Manufacturing Format (3MF) is a 3D printing format that allows design applications to send full-fidelity 3D models to a mix of other applications, platforms, services and printers. The 3MF specification allows companies to focus on innovation, rather than on basic interoperability issues, and it is engineered to avoid the problems associated with other 3D file formats.