This book constitutes the refereed proceedings of the Third International Symposium on Algorithmic Game Theory, SAGT 2010, held in Athens, Greece, in October 2010. The 28 revised full papers presented together with 2 invited lectures were carefully reviewed and selected from 61 submissions. The papers are intended to cover all important areas such as solution concepts, game classes, computation of equilibria and market equilibria, convergence and learning in games, complexity classes in game theory, algorithmic aspects of fixed-point theorems, mechanisms, incentives and coalitions, cost-sharing algorithms, computational problems in economics, finance, decision theory and pricing, computational social choice, auction algorithms, price of anarchy and its relatives, representations of games and their complexity, network formation on the internet, congestion, routing and network design and formation games, game-theoretic approaches to networking problems, and computational social choice.