Sony ATRAC3 audio files can be exported to FLAC format using Sound Forge program.
Convert Sony ATRAC3 audio file to Audio files encoded by Flac - free lossless audio codec.
We found one software available in the database for this conversion.
Microsoft Windows software - convert at3 to flac on Windows
A Digital audio editing software from Sony
Convert from at3 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.