What are three motifs and one theme in John Steinbeck's Of Mice and Men?

One theme that prevails in Of Mice and Men is that, during the Great Depression, the American Dream failed in its idealism in an unfeeling, materialistic society.
With the setting of the 1930s, George and Lennie find themselves among the thousands of disenfranchised, itinerant men who went to California for jobs on ranches and farms. In their hearts, they entertain a hope of owning a little farm of their own where they can raise some animals and crops and "live off the land." According to Steinbeck, this dream is constantly threatened by capitalism and its class system. For instance, Curley is pugnacious and arrogant because, as the boss's son, he is confident enough to antagonize any of the men. In Scene 2, Curley approaches Lennie and goes into the slight crouch of a boxer and asks, "You the new guys the old man was waitin' for?" When George answers for Lennie, Curley "lashed his body around" and says,

"By Christ, he's gotta talk when he's spoke to. What the hell are you getting' into it for?"
"We travel together," said George coldly.
"Oh, so it's that way." 
George was tense and motionless. "Yeah, it's that way."

Similarly, the mechanic Carlson bullies others because he is more confident in his position than the field hands who can be easily replaced. He shows little consideration for others' feelings. When he comes into the bunkhouse in Scene 3, he exclaims,

God, awmighty, that dog stinks. Get him outa here, Candy. I don't know nothin' that stinks as bad as an old dog. You gotta get him out.

With no regard for Candy's feelings, Carlson insists he be allowed to shoot the dog, and does so with Slim's approval.
George and Lennie try to "get a stake," but it is hard for them to save money. When Candy asks to join in on the ranch and offers to put up $300 he has in the bank, George begins to believe it might be a possibility to have a little place of their own. Lennie, the character Steinbeck created to represent the frustration and helplessness of men in a capitalistic society, inadvertently affects the failure of this dream.
Three motifs in Of Mice and Men are fraternity, loneliness/ insecurity, and women as temptresses.
—FraternityGeorge, Lennie, Candy, and Crooks's talk of working together toward having a place of their own almost becomes a reality. Whenever George recites their dream, they point with pride to their friendship:

Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. . . With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us (Scene 1).

Crooks, who is marginalized and made to stay in the barn, talks with Lennie. He speaks of his loneliness and explains the importance of fraternity:

A guy goes nuts if he ain't got nobody. Don't make no difference who the guy is, long's he's with you (Scene 4).

Crooks adds that a man needs someone else by whom to "measure" himself or tell him what he has seen.
—Loneliness/ Insecurity
The many bindle stiffs and dispossessed men of the Depression travel across the country from job to job. They have no stable home, and few even have friends. When George and Lennie arrive at the ranch, George looks cautiously around the bunkhouse. Candy, the old swamper, shows them their bunks. He is careful about what he says, but tells them Curley does not give anyone a chance.
Crooks is very lonely because he is forced to stay in the barn. While he is cruel to Lenny when he first enters the barn and starts into Crooks's room, Crooks soon talks with Lennie, and he even welcomes Candy, who has never entered his room. Crooks says he has only had two visitors the entire time he's lived in the barn.

Guys don't come into a colored man's room very much. Nobody been here but Slim. Slim an' the boss (Scene 4).

Curley's wife, the only woman on the ranch, is very lonely. When she enters the barn in Scene 4, Candy tries to get her to leave, as does Crooks, who tells her "We don't want no trouble." Curley's wife tells the men she, too, is lonely.

Well, I ain't giving you no trouble. Think I don't like to talk to somebody ever' once in a while? Think I like to stick in that house alla time? (Scene 4)

—Women as Temptresses
George warns Lennie against women since they have had to flee the trouble in Weed. After Curley's wife stands in the doorway of the bunkhouse, George again warns Lennie, telling her to stay away from Curley's wife, who he describes as a "piece of jail bait." Indeed, Curley's wife is portrayed as a temptress:

She put her hands behind her back and leaned against the door frame so that her body was thrown forward. . . She smiled archly and twitched her body (Scene 2).

While Curley runs back and forth trying to locate his wife, George comments,

You give me a good whore house every time. . . A guy can go in an' get drunk and get ever'thing outa his system all at once, an' no messes. . . These here jail baits is just set on the trigger of the hoosegow (Scene 4).

When Curley's wife talks to Lennie in the barn, she laughs at his childishness. Thinking he is someone to toy with, she then asks him if he would like to touch her hair, which she claims is very soft.
This suggestion causes Lennie to stroke her hair with his powerful hands. When she becomes frightened by his strength and struggles, Lennie panics and tries to restrain her from crying out, accidentally breaking her neck. 
Certainly, Lennie and George are unfortunate men in an unfortunate and cruel time in America.