Why Coreelec, Alexelec have Python 3? Only you cannot use Python 3? why?
Installing both Python3 (that cannot be used by Kodi) side-by-side with Python2 (which is used by Kodi) makes no sense for a size optimised distribution like LibreELEC that targets Kodi - including Python3 at this time would benefit almost nobody and be a waste of space for everyone else. LibreELEC will replace Python2 with Python3 during the Kodi 19 development cycle.