Many, completely different, .prt files exists but in most cases these contain 3D models of parts and other mechanical components and these can be actually exported to 3D Manufacturing Format (.3mf file) just fine. It's just a matter of finding the right software for your particular .prt file. Most .prt files from Internet come from SolidWorks actually, so this is the software you have to use.
Updated: August 4, 2023