This book constitutes the proceedings of the Third International Symposium on Agent and Multi-Agent Systems: Technologies and Applications, held in Uppsala, Sweden, during June 3-5, 2009. The 86 papers contained in this volume were carefully reviewed and selected from numerous submissions. There are 13 main tracks covering the methodology and applications of agent and multi-agent systems and 8 special sessions on specific topics within the field. The papers are divided in topical sections on social and organizational structures of agents; negotiation protocols; mobile agents and robots; agent design and implementation; e-commerce; simulation systems and game systems; agent systems and ontologies; agents for network systems; communication and agent learning systems; Web services and semantic Web; self-organization in multi-agent systems; management and e-business; mobile and intelligent agents for networks and services; engineering interaction protocols; agent-based simulation, decision making and systems optimization; digital economy; agent-based optimization (ABO2009); distributed systems and artificial intelligence applications.