Hello,
Can Scrivener 3 for windows natively read:
*) all scrivener 1.9.x Windows projects?
*) all scrivener 2.x Mac projects?
*) all scrivener 3.x Mac and Windows projects?
without any processing by the other versions of Scrivener?
Thank you.
Yes.
Note that it will automatically convert the first two to Scrivener 3 format, whereupon they will not be readable by their respective original creators.
Katherine
Great - that was my understanding as well.
Bad - my SSD is corrupting my files. ~12 corrupt Scriv projects. [ a few other Docx/Jpgs/etc as well ] 
Having Dropbox, Onedrive, and Google drive on the same SSD is not necessarily a good idea. [ different directories of course ]
Luckily I have a fourth level of backup to network connected HD, hopefully that will help get some work back.
<happy 2018. sad sigh>