I need 3 examples of metaphors from Lord of the Flies.

William Golding sprinkles metaphors throughout his writing, and the other answers give some good examples. Here are three more.

In Chapter 5, as Ralph follows a narrow path to the meeting place, Golding writes,

"He found himself understanding the wearismomeness of this life, where every path was an improvisation and a considerable part of one's waking life was spent watching one's feet." 

Here Golding draws a comparison between the careful way Ralph has to walk along the jungle path to the careful way the boys have to think about and make decisions in order to not fall into danger. He realizes they are not doing a good job of "staying on course," or "walking the straight and narrow path" of civilized society.

Golding describes Ralph's lack of clear thinking in a variety of ways. In Chapter 7, he relates the conflicting voices Ralph hears in his head and says "the darkness and desperate enterprise gave the night a kind of dentist's chair unreality." It is a stark comparison to bring in such a distant image from the far removed, technologically advanced society they used to live in, but it shows that Ralph's thinking is numbed or drugged with fear in this scene.

In the final chapter, Golding describes Ralph's wavering sense of sanity and logic as "the curtain that might waver in his brain, blacking out the sense of danger, making a simpleton of him." In this way he compares Ralph's inability to think clearly to a curtain that can hide one's view and blind one to necessary information. 

All three of these metaphors make intangible thought processes easier to understand by comparing them with physical objects and experiences.