It sounds like you are mainly looking for help with forming a thesis that emphasizes the theme of humanity in The Road. I understand your frustration. Thesis writing is hard. I recommend using either of the following thesis formats.
The first format is statement-and-reason(s). This might work well for you since you know that you want to have three paragraphs of support. The thesis would provide an argumentative statement and then three pieces of support, and the body paragraphs would explain each piece. The thesis statement might look something like the following statement.
"The Road displays the theme of humanity through the man, the boy, and the final stranger."
That thesis is a functional thesis. It makes a statement and then tells readers what you plan on using for support. It's not a beautiful and flowery thesis, but it doesn't have to be. It makes an argument and provides readers a preview of the information that is going to be used for support.
The other thesis format begins with the word "although." This word sets up a two-part statement. The first part is an argument that runs counter to your actual argument, which is included in the second half of the sentence. This thesis format creates a thesis that looks like the following thesis statement.
"Although The Road presents readers with a world that seems devoid of humanity, certain characters do give readers hope that not all humanity in the world has been lost."
Functionally, this thesis statement is no different than the first statement. It still makes an argument that certain characters are used to display the theme of humanity. The only additional piece of the thesis statement is that you are now required to explain to readers how the world in the book also shows a lack of humanity.