I have a Xiaomi Mi3 running an Android 6.0 Cyanogenmod version and the back button doesn't work anymore.
In which way can I implement the standard touch buttons on the screen (home, back and menu) in order to use them?
/system/build.prop and add the line
qemu.hw.mainkeys=0. Save and reboot. Should you want to revert, change that "0" to "1".
Alternatively, use a non-native solution such as LMT Launcher.