While the first answer does a really good job of talking about the characteristics of these three characters, I want to talk a little about how they add to the story. I think that the three of them are in the story because they provide a good way to break Buck down and to get him to the point where he can meet up with Thornton.
I think that Buck needs to get to a place where he loves a human being again. He presumably loved the people he lived with in California, but after that he did not love. The three characters you mention treat Buck so badly that he practically dies and needs to be nursed back to health by Thornton. That causes him to love Thornton so much that he is hit very hard when Thornton is killed. Because he is hit so hard and because he is so angry at the Indians, Buck loses all connection to civilization and goes back to being the wild beast that his ancestors were.
The characters you mention, then, are included in the story as a way to knock Buck off the perch he held as lead dog for the mail sled. They treat him evilly so that he can really come to appreciate and love Thornton. This advances the plot that London had in mind for this book.