The characters of Candy, Lennie, and Crooks are all considered outcasts, in one way or another, in Steinbeck's novel Of Mice and Men.
First, Candy is considered an outcast given that he has simply outworn his ability to function as a worker on the farm. This is mirrored by the fact that his dog is put down for being too old. Candy can be seen in the same way which the dog is: useless and beyond help. The other ranchers' insistence that the dog needs to be put down, based upon his uselessness. Candy recognizes the fact that, one day, he will be seen as useless and discarded as well. Therefore, he is considered an outcast given his uselessness (though the men do not admit it yet--it is coming).
Lennie is an outcast given his diminished mental capacity. His presence on the ranch would not be allowed if George was not there to look out for him and insure that he did what was expected. The fact that he is an outcast is also seen when the men go after him (after he murders Curley's wife) instead of protecting him (as they would if he were a true member of the ranchers' family).
Lastly, Crooks is seen as an outcast for two reasons. First, Crooks is black. Therefore, he is not regarded as having the same worth as a white worker. Second, Crooks has been injured. His ability to perform the duties expected of him have been reduced. Given that he had dedicated his life to the ranch is the only reason he has been allowed to stay.
The biggest reason these three men in particular are considered outcasts is because they have something about them that makes them undesirable. Lennie is mentally handicapped, Candy is very old and only has one hand, and Crooks is black. That is the gist of it. It's very unfair for these three men because all those reasons are completely out of control, but hey it's life.