Hello,
Please note that generating PDF/A-3 documents using the conversion plugin based on LibreOffice is available since the release of phpdocx 14.0 (https://www.phpdocx.com/news/post/phpdocx-v14-release-notes/229). We recommend you update to the latest version available and check the macros-libreoffice folder included the package to know how to add macros to the conversion plugin based on LibreOffice (in this same folder you can find all available macros).
Regards.