Old Sony ATRAC3 audio files (.at3) are unfortunately not usually supported in standard multimedia players, which makes things hard, especially for just few quick exports or conversions. The only way how to export these files to some other format, such as MP3, is to find someone with copy of Sound Forge or Sony SonicStage and do that export for you. Perhaps online community can be of help here, it is likely that there are still communities for old Sony software around. But otherwise, no such thing as at3 to mp3 converter exists.
Updated: July 21, 2023