Candy is old and in this manual labor job, he is becoming increasingly useless. Consequently, he is treated as such. When Carlson shoots Candy's dog, this is a foreshadowing moment that Candy will be soon no longer be useful. That's why Candy is so keen on getting together with George and Lennie and their dream of owning a farm.
Crooks is an outcast because he is black. He is physically separated from the other ranch hands. They sleep in the bunkhouse; Crooks sleeps in a shed. Crooks is also not invited to go with the other men to the bar/whorehouse. When Candy tells Crooks that Lennie and George plan to get a farm, he is skeptical at first. But he sees this as a possible escape, especially considering the dream is coming from two fellow outcasts: Lennie and Candy. However, after being humiliated by Curley's wife at the end of Chapter 4, Crooks reverts back to his isolated, hopeless state and tells Candy that he is not interested in their farm after all:
"Well, jus' forget it," said Crooks. "I didn't mean it. Jus' foolin. I wouldn' want to go no place like that."
Lennie is an outcast because of his mental handicap. He is therefore limited to manual labor which, being a limitation, is also a kind of outcasting. However, Lennie doesn't even fit in with the other laborers (with the possible exceptions of Candy and Crooks who he seems to get along with at least in Chapter 4). Lennie is innocent because he doesn't know any better. But his powerful strength combined with his poor social skills makes him a danger to others. George tries to protect Lennie and create situations where Lennie can fit in but Lennie always ends up saying something wrong, breaking some social code, or hurting someone.