In Chapter Three of Steinbeck's Of Mice and Men, several things may be seen as symbolic.
At the beginning of the chapter, when George and Slim enter the darkened bunkhouse, Slim turns on a lamp that immediately illuminates the area in which they stand, but does not reach the outlying areas of the room.
The table was brilliant with light, and the cone of the shade threw its brightness straight downward, leaving the corners of the bunk house still in dusk.
This solitary light is symbolic of hope. While George and Lennie dream of owning their own place one day (a light of hope in the dark times of the Depression), there is a growing realization that the economic structures of the society in which they live—the preservation of poverty and hopelessness (symbolized by the far-reaching darkness in the corners of the room)—will never allow their dream to come to fruition.
The symbol that stands out most for me is the sense of frustration George experiences at always having to work for someone else, while under other circumstances he might have his own place and enjoy the fruits of his labors.
An' I ain't so bright neither, or I wouldn't be buckin' barley for my fifty and found. If I was bright, if I was even a little bit smart, I'd have my own little place, an' I'd be bringin' in my own crops, 'stead of doin' all the work and not getting what comes up outa the ground.
This exchange between George and Slim symbolizes the American dream of success and prosperity that is at that time available to only a very few. The common man has no real chance at upward mobility.
It can also be noted that people who are struggling are looking for someone they can admire and respect: someone who is strong and evokes a sense of well-being among the less fortunate. Because the government and society are unwilling (or unable) to take care of the common man, people search for another hero. We see this hope symbolized by the Western magazine Whit brings into the bunkhouse.
Though the tales in the magazines are unreal, and something the ranch men publicly scoff at, they offer heroes in whom the men secretly believe.
Slim is symbolic of the hero for which the men are searching.
Respected by all, Slim is a master at his trade and has moral authority over the other men. Quiet, grave, and perceptive, he invites confidence by accepting people as they are (eNotes).
Like the darkness that the table lamp cannot reach, there is only so much Slim can do.
Additionally, Steinbeck uses Candy (the "crippled. . . stable swamper") and his old dog to symbolize (and parallel) George and Lennie. While the dog is old and smells, Candy is devoted to him—the dog is his only friend. Lennie is like the dog in that he is unable to function in a manner that is socially acceptable; like the old and ailing dog, Lennie is of little use to society because of his child-like mind. George tries to protect Lennie, and Candy tries to protect his dog. These two parallels reflect a time-honored code of caring for those who depend upon you and need a champion. Candy has had the dog since it was a pup and wants to protect him. Candy is a "charity case," however, and cannot afford to offend anyone at the ranch for fear of losing his job. Carlton argues, relentlessly pushing Candy to submit:
"Got no teeth," he said. "He's all stiff with rheumatism. He ain't no good to you, Candy. An' he ain't no good to himself."
Candy is eventually coerced into giving up the dog. Carlton takes the animal outside and kills it, believing he is putting it out of its misery. George and Lennie have also been together quite a while—since Lennie's last living relative died and George became his caregiver. As the story progresses, the similarity between Candy and George in their roles as caretakers will become even more apparent.