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. 

Hal and Charles are the representations of everything evil and bad that could happen to a sled dog. They don't understand how to treat the dogs in terms of physical care and feeding, don't realize the limits of the dog team and make unrealistic demands of them, and don't care about their investment in the dogs, sled, and equipment. Their only motivation is getting to the gold and obtaining as much as possible.

Mercedes isn't actively evil, but her actions are ultimately harmful. She thinks she is being kind by giving the dogs more food, until the supply is gone and there is no more for any of the dogs. She insists on riding in the sled at all times, forcing the dogs to pull her weight along with the rest of the gear packed for the trip into the Yukon.