The three cultures referred to in the title are: the humanities, the sciences, and the social sciences. The theme of the lecture is comparative librarianship. The comparisons are of public library services in different countries, how they are similar and how they differ. His emphasis is on British libraries with which he is most familiar. The beginning of the Sheffield Interchange Organization (SINTO) is recounted as the first library cooperation scheme in Britain. The establishment of the National Lending Library of Science and Technology (NLLST) and its success is described. A criticism of the Dainton Committee is that it has leaned heavily towards the needs of scientists and technologists and has not paid sufficient attention to the requirements of the social sciences and humanities. The public libraries, with the help of national, academic and special libraries, through the assistance of organized schemes of cooperation, and with the positive aid accorded by plans of subject specialization, can and must go on being instruments of education serving all branches of culture. (NH)