This book constitutes the refereed proceedings of the Third International Workshop on Mathematical Methods, Models, and Architectures for Computer Network Security, MMM-ACNS 2005, held in St. Petersburg, Russia in September 2005. The 25 revised full papers and 12 revised short papers presented together with 5 invited papers were carefully reviewed and selected from a total of 85 submissions. The papers are organized in topical sections on mathematical models, architectures and protocols for computer network security, authentication, authorization and access control, information flow analysis, covert channels and trust management, security policy and operating system security, threat modeling, vulnerability assessment and network forensics, and intrusion detection.