Sir Alfred Joseph Hitchcock was a very famous British film director and producer. He was converted into a partly fictious character, by Robert Arthur, with the same name (Alfred Hitchcock), who introduced the mystery or case. After solving the mystery, the sleuths reported to Hitchcock, and sometimes gave him a momento of their case. The introduction that was written before the novellas started, was, in fact, written as if Alfred Hitchcock was writing it in his introduction of the sleuths' mysteries.
After his death in real life, due to some reasons, his fictitious character was also replaced by Hector Sebastian, a retired detective. The film director, in the first mystery of the sleuths, was replaced by Reginald Clarke.
His temporary secretary, during the time when The Three Investigators started their detective firm, was Henrietta Larson, one of their minor enemies. She does not come up in any of the later books.