How did Marxism, Communism and Fascism impact literature of the 20th Century?
It would require completing detailed research, formulating a thesis, and then writing a book or dissertation to really answer this question, but some general observations and specific examples come to mind. Political philosophies, when they are implemented in society, are always reflected and often examined in the literature of the period. For fiction writers, political movements often become elements of plot or are addressed thematically through their impact upon the writers' characters. Through these methods, writers either embrace or reject the political philosophies of their day.
Twentieth-Century literature is a very comprehensive category, but consider American literature in the 20th Century, and it becomes clear that Marxism, Communism, and Fascism did influence the thinking and writing of some major authors of the time. In various novels, these political philosophies were addressed, for instance, by John Dos Passos, John Steinbeck, and Ernest Hemingway.
In his trilogy, U.S.A., Dos Passos attacked the philosophy and practice of capitalism, presenting it as an immoral political force that destroyed individual lives and corrupted the country. When The Grapes of Wrath was published, John Steinbeck was accused of being a communist for his condemnation of the capitalist financial system, particularly the banking system, that he depicts as having created the Great Depression, destroyed American lives, and then profited from their misery. His novel, Of Mice and Men, also made a strong political statement in showing the lives of dispossessed Americans, those without a home who live on the fringes of American society, shut out from ever achieving the American Dream. Finally, in Hemingway's novel, For Whom the Bell Tolls, his protagonist, Robert Jordan, joins forces with a small band of Spanish Loyalists fighting the fascists; Jordan chooses to die to save the others, having developed a strong sense of community and brotherhood.
In terms of 20th-Century English literature, Orwell's 1984 presents a chilling vision of facism in the extreme imposed upon citizens.