This volume contains the 12 papers presented at the WISTP 2009 conference, held in Brussels, Belgium in September 2009. WISTP 2009 was the third int- national workshop devoted to information security theory and practice. WISTP 2009 built on the successful WISTP 2007 and 2008 conferences, held in Heraklion, Crete, Greece and Seville, Spain in May 2007 and May 2008, - spectively. The proceedings of WISTP 2007 and WISTP 2008 were published as volumes 4462 and 5019 of the Lecture Notes in Computer Science series. This workshop received the following support: - Co-sponsored by IFIP WG 11. 2 Small...