The fact that Freud's hypotheses and theories as he performed them don't stand up to modern scientific scrutiny does not mean he is otherwise useless as in the field of psychoanalysis[0], which has mainstream appeal and respect in the humanities and has largely moved on from Freud himself[1]. Likewise for Marx, who is both popular among heterodox economists and sociologists (since he's a founder of that discipline), not to mention political economy, all three of these fields are huge and produce a mass of scholarship every year - along with the discipline of critical theory which can be credited to the authors I mentioned in my top level post in this thread. There is no feminist scholarship which does not at least take into account critical theory, just as there is no aesthetics which does not either. Nietzche is one of the most influential philosophers of modernity, I'm not sure what's supposed to be so wrong about him.
You could even reject all of their substantive propositions and still respect them and the culture they helped inspire[2], which is the culture of suspicion - not taking things for granted or at face value, repudiating all notions of "common sense" and as Marx put it, "ruthless criticism of all that exists".
"Having mainstream appeal" does not mean legitimacy as a scientific discipline. Psychoanalysis is about as useful as alcoholic anonymous' 12 step program: it's a terrific placebo. Mainstream theories are largely discredited by neuroscience and psychiatry, but it turns out having people just sit on a couch and self-reflect is usually beneficial regardless. But if you want to understand the how's and why's of the human mind with scientific rigor, then do not look to anything in the field of psychoanalysis.
Very few people use psychoanalysis as if it were scientific, even its practitioners. There are also arguments that with some effort in the field it could graduate to science. In general, though, the Popperian criteria for science are not the end of knowledge.
It would be quite a feat to actually read Nietszche and come to that conclusion. I haven't read enough Freud or Marx to be so confident of their cases.
The same goes for Marx and Nietzsche.