Steps to upgrade from 2 to 3

This might fail and break your site, so please use a copy of your production site in order to test it first!
When in production put your site in maintenance mode!

1. Download Feed Import 3 module and place it outside of your drupal site
2. Download and install feed_import_upgrade module (this will do the migration process)
3. Navigate to admin/config/services/feed_import/upgrade (keep this page in a separate window or tab)
4. Disable cron import from Feed Import settings
5. Be sure that no import is currently running
6. Click on the Proceed button and if everything is ok you'll be redirected to step 2
7. Disable and uninstall Feed Import 2 module
8. Delete feed_import module directory
9. Copy Feed Import 3 module and install it
10. Click on the Proceed button and if everything is ok you'll be redirected to step 3
11. Uninstall feed_import_upgrade module and delete the module directory
12. Now the migration is done and you can check your feed items

Notes: you'll have to rewrite the paths for CSV/JSON feeds since they are not compatible with version 3.
If you are having problems with the upgrade please create a new issue for Feed Import (as support request).