Good information, Henryk_Malysz.
This allowed me to find a solution to turn the M3 directly with a Shutdown button.
I missed the ROOT access to the M3.
Thank you for this information.
My tutorial in french :
http://castman.fr/wordpress/tutoriels-sur-les-tablettes-smartphones-micro-ordinateurs-firmware-google-play-reset-wifi/banana-pi-m3-rooter-android-pour-leteindre-avec-le-bouton-shutdown-cest-du-bonus/