This book constitutes the refereed proceedings from the Third International Workshop on Graphical Models for Security, GraMSec 2016, held in Lisbon, Portugal, in June 2016. The 9 papers presented in this volume were carefully reviewed and selected from 23 submissions. The volume also contains the invited talk by Xinming Ou. GraMSec contributes to the development of well-founded graphical security models, efficient algorithms for their analysis, as well as methodologies for their practical usage. .