The three sessions needed to eliminate 84-90% of PTSD diagnosis in single trauma victims may be all that is needed for some clients, and only the beginning for others. The eight phases of EMDR treatment should be completed. However, for some clients the total clinical complaint is the PTSD symptomatology. For other clients, eliminating the PTSD symptoms and treating the trauma is the equivalent of removing the quilt from the mattress. Then you get to see all the other facets that need to be addressed--such as the relationship difficulty, family problems, career problems, etc.
For multiple abuse victims suffering from PTSD three sessions will generally not be sufficient to complete treatment of all the pertinent memories. In addition, the more complex the presentation, the more targets will be necessary. Generally, the EMDR clinician will work on the basis of a three pronged protocol: (1) reprocess the past events that set the groundwork for the dysfunction;(2) reprocess the present conditions that stimulate the dysfunctional responses; (3) incorporate the skills and behaviors needed for appropriate future action.
A good overview of the EMDR treatment for a variety of presenting complaints is the book EMDR (Shapiro & Forrest, 1997, BasicBooks). Case narratives and transcripts can give a taste of the evolution of the positive treatment effects and the wide range of application. Basically, the number of sessions depends upon the needs of the individual client.