Multipart 3mf file, need to extract one of those parts for printing.
If your 3D file has separate objects and vertices and are just merged together then do the following:


Import into Magics;
Select part (in the part list menu).
Right click (in the part list menu) and select Shell to parts;
(Be very careful here) Select the area you want to isolate with the translate tool. If you don't know how to do this just left click hold and drag the mouse to select an area inside a box;
Use translate tool to move it a little ahead from the original grouped object (just drag on any axis);
Right click (in the part list menu) and select merge part.


Should work out fine. Tell me if you get stuck.

This can also be done on Netfabb. Not on SolidWorks.