Much torment has arisen from people perceiving GNS as a labelling device. Used properly, the terms apply only to decisions, not to whole persons nor to whole games. To be absolutely clear, to say that a person is (for example) Gamist, is only shorthand for saying, "This person tends to make role-playing decisions in line with Gamist goals." Similarly, to say that an RPG is (for example) Gamist, is only shorthand for saying, "This RPG's content facilitates Gamist concerns and decision-making." For better or for worse, both of these forms of shorthand are common.
For a given instance of play, the three modes are exclusive in application. When someone tells me that their role-playing is "all three," what I see from them is this: features of (say) two of the goals appear in concert with, or in service to, the main one, but two or more fully-prioritized goals are not present at the same time. So in the course of Narrativist or Simulationist play, moments or aspects of competition that contribute to the main goal are not Gamism. In the course of Gamist or Simulationist play, moments of thematic commentary that contribute to the main goal are not Narrativism. In the course of Narrativist or Gamist play, moments of attention to plausibility that contribute to the main goal are not Simulationism. The primary and not to be compromised goal is what it is for a given instance of play. The actual time or activity of an "instance" is necessarily left ambiguous.
Over a greater period of time, across many instances of play, some people tend to cluster their decisions and interests around one of the three goals. Other people vary across the goals, but even they admit that they stay focused, or prioritize, for a given instance.