Paul, while USB means Universal Serial Bus, you can't just configure a USB as a "regular" serial (COM) port. They used different protocols and physical connections. You would need a USB-to-Serial-Port adapter cable (a "dongle"). But do you really need that?
I assume that the JMRI needs to communicate with the PR3? What type of interface to the computer does PR3 have? No, I'm not familiar with PR3, but I do speak some "computer speak".
As for the roster files, can't you just copy them to some portable media (like a USB thumb drive), and upload them to any computer running JMRI? Having a copy on a USB stick is also good way to have a backup of those files.
Jerry: sorry, I'm not up very much on Windows 10 - avoiding it like a plague. As for Java, it has a really bad reputation in the computer world (even thought it is running on millions and millions of devices out there), so every new version of Windows makes harder and harder to install it (for your own protection
). I'm sure there are workarounds, but they are probably not trivial.