system
Given the following post, respond as if you're commenting on Reddit:
human
What is modernism, postmodernism and post-postmodernism?

And how do these movements relate to the previous movements (like those before modernism). I feel like this is a very broad question (as modernism in art might not be modernism in something else). But I genuinely want to know if someone's codified all of the stuff, like what were the common elements.
gpt
In philosophy it's common to think of "modernist" philosophers as those who begin to break with 19th century thought and belief– so, essentially, it's post-Enlightenment thought. An example, then, of a modernist philosopher might be Nietzsche, though confusingly he is also claimed by postmodernism depending on who you read. This suggests that modernism can be seen as a sensibility rather than a time period– which is an additional complication of classification. By contrast, in literature and the arts, Modernism is a post-Victorian movement involving, primarily, the explosion of form. Confusingly, none of the above would have considered themselves modernists– it's a retrospective label. One prominent aspect of modernism is structuralism– which is important because it's one of the key things critiqued by the PoMos. 
          
Postmodernism is generally thought to begin after the Second World War, and was first identified in the writings of figures like Lyotard, Jameson and Baudrillard. By this point in history, philosophy and the arts had atomised so much that there are countless strands of thought and talking of them cohesively is a difficult task. However, postmodernist thought and art is characterised by a few things: incredulity towards meta narratives, a foregrounding of the polysemous nature of language and signification, and an explosion of the traditional binary of high and low culture.
         
If modernism is reacting to Victorian ideas and values, then postmodernism reacts to modernist ideas and values. Post-pomo does the same again, critiquing postmodernism for its various excesses etc.
        
Hope this helps.

edit: cleaned up numerous confusing phrasings.