Number Three

In the novel The Big Four (and the short stories making up the novel), Number Three is the designation given by the British secret service agent Mayerling for the third member of the leadership of the highly secretive and powerful internation crime syndicate the Big Four. Mayerling knew that Number Three was a woman, and her nationality was French. It was possible that she might be one of the sirens of the "demi-monde" (a courtesan who moved in high social circles) but nothing definite was known about her.

Much later, Poirot discovered that Number Three's background was totally different, and that she was actually the famous French scientist Madame Olivier.