This volume contains the proceedings of the Third International Conference on Network Control and Engineering for Quality of Service, Security and Mobility (Net-Con'2004), celebrated in Palma de Mallorca (Illes Balears, Spain) during November 2-5, 2004. This IFIP TC6 Conference was organized by the Universitat de les Illes Balears and sponsored by the following Working Groups: WG6.2 (Network and Internetwork Architectures), WG6.6 (Management of Networks and Distributed Systems), WG6.7 (Smart Networks) and WG6.8 (Mobile and Wireless Communications). The rapid evolution of the networking industry introduces new exciting challenges that need to be explored by the research community. The adoption of Internet as the global network infrastructure places the issue of quality of service among one of the hot topics nowadays: a huge diversity of applications with quite different service requirements must be supported over a basic core of prools. Also, the open and uncontrolled nature of Internet enforces the need to guarantee secure transactions among users, thus placing security as another hot topic. Finally, the explosion of mobility and its integration as part of the global infrastructure are probably now the most challenging issues in the networking field.