The easiest way to achieve 3mf to jfif conversion, i.e. 2D export of 3D model exported as .3mf file to a JPEG Interchange format (.jfif) is to use the Paint 3D app available for Windows 10, which you can get from Microsoft Store for free.
Once you have opened your .3mf file in Paint 3D, simply click on the folder icon in the upper-left corner and use the Export file function to convert 3mf to jfif. This will create 2D picture of your model.