What are three motifs and one theme found in Of Mice and Men?

A motif is literally anything that repeats in a novel. It can be an image, a metaphor, a symbol, or an idea. In Of Mice and Men, John Steinbeck employs several motifs to help create meaning.
1. Motif: Power
Throughout the novel, there is the repeated use of the idea of power. There are two types of power, specifically: economic and physical. Lennie represents the physical power present in the working men in California. With this brute strength, Lennie literally crushes Curley's hand and snaps Curley's wife's neck with little effort.
Still, those with money exert economic power over the working men. When they arrive at the ranch, George tells Lennie not to "say a word" to the boss because they need the money to live. As the boss's son, Curley exerts power over all the men in the bunkhouse. His wife exerts a different kind of power over the men in the bunkhouse, but it is still based on the power exerted by those with money.
2. Motif: Hands
Throughout the novella, there are many descriptions of hands. Curley's is probably the most memorable, as he keeps his left hand in a glove full of vaseline so it is "soft for his wife." When Candy lost his hand on the ranch, he lost any ability to chase after his dream of owning his own piece of land. When Slim encourages Candy to shoot his old, mangy dog out of his misery, he sits in the bunkhouse and "gazed at [Candy] for a moment and then looked down at his hands." Curley's hand is also destroyed when Lennie crushes it. 
Hands are important because they are necessary for a lot of human actions. Steinbeck mentions the hands of many characters because they are symbols of power (the idea mentioned above). Lennie's hands symbolize his brute strength; Candy's, his uselessness; Curley's, his softness; Slim's, his position as the decision-maker and, thus, the one who suffers from the most guilt.
3. Man vs. Nature
One of the primary conflicts in Of Mice and Men is the idea of man versus nature. The opening two paragraphs of the novella show this. In the first paragraph, Steinbeck uses peaceful words to show how nature thrives in the absence of man. He writes that the Salinas River "runs deep and green" and the "golden foothill slopes curve up to the strong and rocky Gabilan Mountains." Animals—lizards, rabbits, and dogs—enjoy nature undisturbed.
In the second paragraph, the presence of man is made clear. The path has been "beaten hard by boys coming down from the ranches." There is an ash pile below a sycamore, and a tree has been "worn smooth by men who have sat on it."
Throughout the novella, nature alone is peaceful, but it is disturbed immensely by the introduction of man.
Theme: Every man or woman is alone regardless of the choices he or she makes.
One of the characters I always feel sorry for is Curley's wife. I think she's the loneliest character in the book. As the only woman in the novella, she's also the only character in the book who makes multiple appearances, but remains nameless. She chose to marry Curley, but the two never appear together in the book. Instead, they are always just missing each other. Despite her choice to marry Curley, she feels absolutely alone. She admits, "I don' like Curley. He ain't a nice fella." When she dies, she dies alone, buried under some hay. 
Every character who chooses a partner, ends up completely alone by the end. Lennie and George are another example of this, as George shoots Lennie at the end of the novella so he does not have to face Curley's wrath.