Each of the novels places the characters in poor situations, so they all compare to each other in this regard. The reader becomes sympathetic to them because of their plight, and they want them to win. Unfortunately, because of society at the time, for most of the characters, that is not possible. Steinbeck's account of the Joads leaves them in a terrible situation by the end of the book, yet they somehow remain hopeful. Steinbeck is looking at the American people as a whole, and how, when the times are the worst, they still hang on to hope.
As for social impetus, the books did spark change. "The Jungle" actually helped form the first department in Washington to deal with food safety, the Federal Department of Agriculture, which now oversees food safety. Stowe's work did not end slavery single-handedly, but it brought the problem to the attention of the public and helped gain worldwide support...
Steinbeck's book did not end the Great Depression, but it brought attention to the plight of the people who were most affected by it and showed how they were living, which brought sympathy and some measure of relief for them. By writing about these issues, the writers made people more aware of them, and that helped end practices that should not have been occurring.
In conclusion, Steinbeck won the Pulitzer Prize for this novel, which shows how it resonated in the writing community. That is one of the highest honors a writer can achieve, and it shows how powerful this book really is, both in social and literary terms. "The Grapes of Wrath" is a great novel, and compared to the other two, it is superior, if only because Steinbeck was a superior writer.
References
Steinbeck, John. The Grapes of Wrath. New York: Penguin Books, 2002.
