AA3 format was discontinued many years back and not that many programs nowadays support it. Your best luck for converting AA3 to MP3 would be to use some of the programs that used to support it, like Sound Forge.
There might be more programs capable of exporting AA3 format, but not all of them support MP3. So perhaps try to search for other export choices.Open AA3 file Open MP3 file
Audio data management program for portable devices
A digital audio editing software from Sony