There's a Calibre plugin (KindleUnpack?) that can extract azw3 files directly without requiring intermediate conversion to epub.
Actually, I think Calibre may already have a hidden option to unpack azw3 files.
Edit:
Yep, there's a built-in feature in Calibre for extracting azw3 and epub files called "Unpack book". You just need to add the shortcut to the toolbar and/or right-click/context menu.
The
KindleUnpack plugin can extract both mobi and azw3.