Please note that, in order to truly form a supporting argument, we first need a primary argument or thesis to support. I'm assuming that you have a thesis in mind, which makes this response a bit of a generalized speculation. If, on the other hand, you're approaching this from the perspective that the story itself is an argument, then you might consider looking at material from "To Kill a Mockingbird".
"The Strangers That Came To Town" is fairly similar in structure and theme to "To Kill a Mockingbird" - children are educated about the folly of prejudices by a stern but loving father who sets the example for not only the children, but the rest of the town. There is an abundance of material to work with here; for example, a thesis or supporting arguments might compare the characters in the two stories and justify or reject the facts that led the townspeople to treat the "outsiders" as they do.
If you're confining yourself to "The Strangers", then you might consider focusing on elements such as the ways in which the Duvitch family does and does not communicate their value to the rest of the town, or the way in which they embody American ideals and spirit.
Overall, the best way to identify supporting arguments is to think of how we can demonstrate the truth of your thesis; is there information in the story which directly leads to your statement? Factual rather than inferred reasoning would be best if you're limited to 3 arguments; for example, it would be better to argue that "the Duvitches make other people uncomfortable because their poverty is seen as an embarassment", rather than trying to infer exactly why poverty is an embarassment to the rich (although elaborating on this point with some good reasoning and insight will help your position).