The process of Na-3(+) cluster ion fragmentation following excitation by a fast He atom is studied using a theoretical procedure developed earlier. The collision with He leaves Na-3(+) in any of three electronic states, and each of these can fragment into three product channels (Na-2(+)+Na, Na-2+Na+ and Na+Na+Na+). Attention is focused on understanding both the dynamics of the Na-3(+)-He interaction and the post-collisional fragmentation of the excited cluster. Four simple fragmentation mechanisms are proposed to describe the major features of the process. Contributions of these mechanisms to different fragmentation pathways are determined and their dependence on the initial internal energy of the cluster is studied. Fragmentation intensity maps are calculated and good agreement with experiment is obtained.