This book constitutes the refereed proceedings of the Third International Workshop on Post-Quantum Cryptography, PQCrypto 2010, held in Darmstadt, Germany, in May 2010. The 16 revised full papers presented were carefully reviewed and selected from 32 submissions. The papers are organized in topical sections on cryptanalysis of multivariate systems, cryptanalysis of code-based systems, design of encryption schemes, and design of signature schemes.