ss guard charlotte s trained her alsatian to bite at inmates ' private parts .
gisela s locked up to 15 prisoners in a tiny ` standing cell ' for days .
gertrud elli senff revelled in the power of playing ` god ' with people 's lives .
she picked who lived and who was sent to the gas chambers to die .
but all three women - now frail and in their 90s - will never face justice .