what is the meaning of 'three parts' in :"if ever a man was three parts mad with terror, that man's name is Pinner"?

I am quoting from The Memoirs of Sherlock Holmes, The Stockbroker’s Clerk by Arthur Conan Doyle: "There is something which I don’t understand in his manner. If ever a man was three parts mad with terror, that man’s name is Pinner. What can have put the shivers on him.
Is Holmes referring by "three parts" to himself, Watson and Pycroft being the reason of Pinner’s terror or something else and what does Pinner’s name have to do with his being or not being afraid ?