How might one approach a thesis examining early American literature on social injustices such as slavery, women's rights, and the treatment of Native Americans?
I am able to reference early American Literature.
Taken alone, these three are fairly broad topics; combined, they might be difficult or even impossible to coalesce into a paper with a strong, controlling idea. My suggestion would be to select one of the injustices you mention, such as women's rights, and examine American literature on that subject. That would allow you to locate and research different viewpoints, and possibly analyze historical context and how it might or might not have influenced different authors.
If narrowing down to one social injustice is impossible, you might consider locating one literary piece about each topic and comparing and contrasting these pieces. I think this will still be an unwieldy approach, because you will have to determine what criteria or characteristics you wish to examine first; author's purpose, author's personal background, national politics, presidential leadership. . .the list could go on and on and would require some tweaking to really streamline it into something manageable.
Another way to focus your topic might be to select a specific time period in which to survey the literature, but again, I think that would work best if you selected one of the three topics.