In Of Mice and Men, why doesn't George go into town with Whit in chapter 3?
Of Mice and Men is the story of George and Lenny and those with whom they come in contact. Chapter 3 is full of some of the other characters in this novella, but it's also an important foreshadowing of things to come for George and Lenny. When Whit offers George a chance to go with the guys into town to let off some steam, George implies that he might come along; however, we know he'll never go. Earlier in the chapter, George has confided to Slim the story of Lenny and the woman he nearly killed in his fear and innocence. There is just too much potential for trouble in this place for George to leave Lenny alone for even a short outing. George understands that Lenny would do it all again if the circumstances were the same, because he just doesn't know any better. Instead, then, he spends the evening sharing the dream with Crooks and Candy--the dream of some land and a farm of their own. This dream is an escape from the reality of being Lenny's keeper and friend. He'll never be able to go to town and be "one of the guys" as long as Lenny is in the picture. Instead, he's destined for a life among the misfits... unless something changes.
I assume that you are asking about the part in this chapter where Whit is talking about going in to town and visiting a whorehouse. This is the part where he talks about the two whorehouses in town and compares them. When Whit talks about that, George says he does not want to go because he does not want to waste the money.
Each time that Whit talks about going into town, George replies by saying that he and Lennie are trying to "roll up a stake." By that, he means that they are trying to save money to buy that farm that the two of them dream of owning.
To me, this shows that George is dedicated to the idea of the dream that he and Lennie share. He would rather defer pleasure now so that he can have a better chance at the future that the two of them want.