These three characters from Of Mice and Men are misunderstood loners. In the social hierarchy of the ranch, each suffers from a disability that places them in the lowest ranks. Social prejudice (race and age) are also factors that leave them loney.
Lennie is mentally challenged, so many of the others (except George) disregard him. He is a strong worker, which is something workers like Slim appreciate, but Lennie wouldn't be considered one of Slim's friends. He sleeps in the bunkhouse with the rest of the workers, but he is not considerd an equal.
Crooks is the only Black man on the ranch, and he is segregated to the stable to sleep with the horses. He's not allowed into the bunkhouse, even to play cards. Like Lennie, he suffers from a disability that adds to his seclusion. Crooks was once kicked by a horse, which left him with a crooked back; hence the name Crooks.
Candy's disability is that he is one-handed, and so he cannot do the labor that the others do on the ranch. Although he does sleep in the bunkhouse with the others, like Lennie, he is not treated as an equal. He is old, but age is not revered in the world of the ranch. When something gets old (like Candy's dog), it is discarded. Candy feels that it won't be long before he becomes totally useless and then he will be discarded as well.