Lord of the Flies is a novel written by William Golding. It was first published in 1954 and follows the story of young boys who find themselves on a deserted island.
In order to help you answer your question, it is first of all important to point out that a lot of imagery is used at the beginning of chapter 3 in order to describe Jack as he is venturing through the forest, trying to find a path. For example, right at the start of this chapter, we read that Jack was "down like a sprinter," which shows the reader that Jack must have positioned his body very low in order to be able to investigate the floor more closely. He is trying to find the way almost like a dog, by keeping his head close to the ground in order to find hints of a trail. This is why he is also described as "dog-like" at this point.
Jack is very much described as animal-like at this point in the book; for example, the author also tells us that Jack had "flared nostrils," which is also an image that one would most commonly associate with an animal such as a dog. When Jack is startled by the cry of a bird, the author continues with this animal-themed imagery, describing Jack as " ape-like."
You could interpret this use of imagery in many ways. I would suggest perhaps looking into the further development of Jack's character within the book. You could argue, for example, that perhaps Golding chose this kind of imagery because he wanted to show the reader already in chapter 3 the kind of animal-like person Jack is and how he often lacks human qualities such as empathy.