As detailed in our official "end of life" notice for Papers 3, as of December 31st, 2020 we are no longer supporting our previous application - Papers 3 - in any way, including migrations from Papers 3.
Over the years we have offered migration tools to assist users in moving their libraries from Papers 3 to our latest Papers application and Papers 3 no longer runs on the latest versions of MacOS - with the last compatible version being MacOS 10.15 Catalina. Anything newer than MacOS 10.15 will not work with Papers 3. In addition, our Papers 3 to Papers migration tool also does not work on these newer operating systems and, like Papers 3, will no longer be updated.
Though you will not be able to use a migration tool to bring your library over from Papers 3, you still have access to your files.
By default, your Papers 3 library will be stored in Documents on your computer.
Inside "Papers Library", you'll find all of your PDFs grouped into sections:
Your PDFs will be stored in those folders. You can drag and drop those folders into Papers for import, or elsewhere if you prefer.
In addition, while you do not need Papers 3 to launch in order to get the data above, if Papers 3 does launch on your computer, you can also export your Papers 3 library as a .bib or .ris file for import elsewhere: