This volume constitutes the refereed proceedings of the Third IFIP WG 11.2 International Workshop on Information Security Theory and Practice: Smart Devices, Pervasive Systems, and Ubiquitous Networks, WISTP 2009 held in Brussels, Belgium in September 2009. The 12 revised full papers presented were carefully reviewed and selected from 27 submissions for inclusion in the book; they are organized in topical sections on mobility, attacks and secure implementations, performance and security, and cryptography.