This is a great question, because in a lot of ways the story itself confounds sympathy. The trick is to define what we mean when we are trying to be "sympathetic." In general, the most common definition would be that sympathy is the act of feeling sorry for someone else's misfortune. But to feel sorry for someone we must care a little bit about them, and these three characters can make it difficult.
First, Kenny. He's a bit of a jerk, so it's hard to feel sorry for him for much of the story. But in a real way, he is also the most sympathetic character (ironically, considering his jerk-a-tude.) I think that this stems from the fact that even though he's a jerk, that doesn't mean we want to see him die in the back of the truck. He was shot by Tubs for what amounts to a prank (not telling the others he had permission to kill the dog) and in this respect it is possible to feel sorry for him, especially when the other men seem to make so many pit-stops on the way to the hospital and seem largely indifferent to his suffering.
Tubs would make the most obvious character for sympathy, but it's a bit of a false-lead. At first the reader feels bad for Tubs because he is a bit of a buffoon that gets picked on by the others (especially Kenny.) He is out of shape and can't keep up. We can even forgive Tubs shooting him because Tubs was genuinely afraid for his life.
The reason that Tubs fails to be a sympathetic character in the end is because A) He ultimately isn't too concerned with getting Kenny to the hospital, despite the fact that he's erroneously shot him, and B) He admits that much of his weight problem stems from overindulgence (shown in the way he wolfs down the pancakes.) Yes, he got teased in a way that most people can relate to, but in the end he is the cause of his own ridicule. His lack of morality when it comes to Kenny makes it hard to feel bad for him.
Frank is more in the middle. He's not as much of an a-hole as Kenny, but he's not exactly sticking up for Tubs, either, nor does he make any effort to hurry his friend to the hospital (or even remember where the hospital is, for that matter.) We also get the weird confession that he is in love with his children's underage babysitter and is considering abandoning his family in pursuit of her. That alone makes him creepy enough to reject. I suppose in a twisted way it might be possible to find sympathy for him in that he is so lovesick, but it is in such a repugnant way it's hard.
So there you have it. In the end, the character who is outwardly the least likable is also the most sympathetic in the story. This stems not from his character, which appears slightly sadistic, but from the fact that being a foul human being is not enough of a reason to deserve bleeding out in the back of a frozen truck when your fat friend gobbles pancakes in a roadhouse.