flac to at3 conversion is almost certainly searched by users that are looking for a way to turn audio content from FLAC format to Sony ATRAC3 format, something that should be possible with any tool that still supports AT3 format.
Sound Forge from Sony comes to mind, as it can certainly work with FLAC files, but perhaps more software from Sony can be of help as well. In any case, if you need help with ATRAC3, it might be best to reach out either to Sony support or perhaps some suitable online community.
Updated: October 6, 2023