Exporting ratings

Star ratings are not common tags, which means they are not stored in the audio file's tags such as artist/title/album are. Star ratings are proprietary to the software you use.

I don't understand what you mean by "export star ratings", but I get that you want to remove all tracks with a rating below 3 from your Rekordbox collection including the audio file.

Now this is a long shot, but what you want to do is possible is you're on macOS. It involves using some of my tools.

1. Convert your Rekordbox collection to iTunes (or it's successor the Music.app) using the DJ Conversion Utility (DJCU). The conversion to iTunes doesn't modify anything in Rekordbox and DJCU can convert the RB star ratings to iTunes.
2. You'll need another app of mine, the iTunes Collection Tool (iCT). iCT's primary function is to help you fight duplicates in your collection using audio finger printing. But it also comes with a lot of keyboard shortcuts. One of them is removing selected audio file(s) from the iTunes collection including moving the associated audio file to a pre-bin folder. You can then simply delete the audio files form that pre-bin folder. 

in this AMA live stream I demonstrated some of iCT's keyboard shortcuts