The Third International Conference on Algorithmic Decision Theory has been held on November 13 - 15, 2013 at the Université libre de Bruxelles, Belgium. The conference has been organized by Marc Pirlot, Yves De Smet, Patrice Perny, Alexis Tsoukiàs, and Paolo Viappiani. The program included four invited talks, thirty regular talks, and a poster session. The conference covered topics such as social choice, voting, preferences, multiobjective optimisation, preference learning, decision under uncertainty and risk, and multicriteria decision aid. The conference proceedings have been published as Volume 8176 of the Lecture Notes in Artificial Intelligence series. This volume includes twenty-four of the presented papers and the ten papers of the poster session. Six of the presented papers have already been published elsewhere and have not been included in the proceedings.