This paper is intended to sum up the results of the Third International Workshop on Software Engineering for Large-Scale Multi-Agent System (SELMAS 2004) held in Edinburgh, Scotland, May 24--25, 2004, as part of the International Conference on Software Engineering (ICSE 2004). The main purpose of this workshop was to share and pool the collective experience of people, both academics and practitioners, who are actively working on software engineering for large-scale multi-agent systems. The call for papers elicited some 24 submissions, of which 14 papers were accepted for presentation. A set of selected workshop and invited papers are to appear in a Lecture Notes in Computer Science (LNCS, Springer) volume. The workshop consisted of an opening presentation, two keynote talks, four technical sessions of paper presentations, and two panels. During the workshop we informally reviewed ongoing and previous work and debated a number of important issues. The SELMAS 2004 website can be found at <http://www.teccomm.les.inf.puc-rio.br/semas2004>. We begin by presenting an overview of our goals and the workshop structure, and then focus on the workshop technical program.