The story
three (“3”)+emperor+corner. From being a point where three empires once met.
Etymology from Wiktionary, CC BY-SA 4.0.
Three Emperors' Corner
Today it means: A confluence in Black Przemsza river, White Przemsza river, becoming Przemsza river, in what is now Silesian Voivodeship, Poland; once the meeting point of the Empire of Russia, German Empire, Austrian-Hungarian Hapsburg Empire. Full definition →