The German Third Reich (ドイツ第三帝国?) was the regime leading Germany during World War II. They were involved in mysteries like magecraft, even seeking the Holy Grail during the Third Fuyuki Holy Grail War.
Within the world of Fate/stay night, the Third Reich fought against the Imperial Japanese Army for the Greater Grail, but both sides were unsuccessful.
Within the world of Fate/Apocrypha, they were able to steal the Greater Grail in cooperation with Darnic Prestone Yggdmillennia, but he double crossed them for his own gains.
Within the world of Fate/KOHA-ACE, Hitler dies earlier than normal and his body double takes up his name and travels to Japan to compete in the Imperial Capital Holy Grail Strange Story.