This book constitutes the thoroughly refereed post-workshop proceedings of the Third Workshop on Theory of Quantum Computation, Communication, and Cryptography, TQC 2008, held in Tokyo, Japan, in January/February 2008. The 10 revised full papers presented were carefully selected during two rounds of reviewing and improvement. The papers present current original research and focus on theoretical aspects of quantum computation, quantum communication, and quantum cryptography, which are part of a larger interdisciplinary field that casts information science in a quantum mechanical framework.