azw3 to mp3 conversion is usually related to attempt to convert digital book to audio book, i.e. conversion of Amazon Kindle ebooks to MP3 audio.
The most reliably way to do this, would be to use some text-to-speech converter, such as Balabolka, that will generate audio file (.mp3) from text source (.azw3). Usually uses the voices provided by Microsoft, but some programs have their own unique ones.