This book constitutes the refereed proceedings of the Third International Workshop on Intelligent Agents for Telecommunication Applications, IATA'99, held as an IJCAI'99 Workshop in Stockholm, Sweden, in August 1999. The 14 revised full papers presented were carefully reviewed and selected for inclusion in the book. The papers are organized in topical sections on architectures, tools, and languages; network management; electronic commerce; and telecommunication services.