The comparison of Lenny and Curley is one of the most nuanced and intriguing aspects of this book. Lenny appears docile, large but harmless, and as an idiot easily controlled. Curley is agile, small and harmful and incapable of control. Lenny has a hulking physical body but a damaged mind. Curley has a very sharp mind but ends up with a damaged body. They seem wholly dissimilar.
However the same force drives them both. Curley and Lenny derive pleasure from the manipulation, exploitation and destruction of smaller things around them. Lenny must have a soft animal to control and possess to give him pleasure. Curley is seeking to possess and control every aspect of his wife’s life. Lenny uses his faulty mind to try to sneak things past George. Curley uses his soon to be maimed body to exert control over the other ranch hands. Both destroy everything they touch.
Lennie and Curley are from different classes in the American social structure. Curley, as the son of the ranch boss, will one day become a land owner through inheritance and not necessarily through hard work. In the social structure of the ranch, Curley might be considered part of the upper class because he was born into money. Lennie, on the other hand, would be considered part of the lower class, not only because he is poor and travels to find work, but because he is mentally handicapped. Unlike Curley, Lennie will never inherit land, but through hard work, he and George could be able to "get a little place".
Both Lennie and Curley act without thinking. Each reacts to situations rather than taking the time to think things through. Lennie reacts to fear by clenching down on whatever is in his grip (a girl's dress, a mouse, Curley's hand, a puppy, and Curley's wife's hair). Each instance becomes progressively worse throughout the novella. Curley's reactions lead to similarly destructive situations (a poor relationship with the ranch workers and with his wife, a fight with Lennie that leaves him crippled, and murderous intentions when his wife is killed), but it's his anger that drives him.
Lennie and Curly are not two characters who immediately come to mind that share a lot of similarities, they are very different. I think what the two men share most in common is fear. Curley is a fearful coward who hides behind his tough bully facade and position in the ranch. Lennie is afraid that he will disappoint George. He doesn't want George to get upset because then he won't get to "tend the rabbits" on their dream farm.
The men are extremely different. Curley has a mean temperament which is caused by his lack of confidence in everything including his marriage. Lennie is sweet and while he is also unsure of himself, he makes no attempt to mask it. Curley also comes from a different class in society which he believes entitles him to treat others poorly and emphasize others' shortcomings. Lennie isn't even aware of the fact that classes exist and he sees everyone as the person they are, not for what their place in society is.
Chapter 3 has several examples that illustrate the stark differences in each man, but it also demonstrates the fear each man has as well. The entire scene when the fight that ensues between Lennie and Curley occurs is a great example of their shared fear and their differences.