First off, there are 12 chapters in the The Lord of The Flies. So you are summarizing the whole book.
I am going to assume that this is a 3 paragraph essay but it doesn't matter that much. I suggest that you write down the main events of each chapter(maybe one or two sentences each) then take chapters 1-4 and write one to two sentences about each of the chapters. Repeat with 4-8 and 8-12.
Having read the book I will give you an example of how I might go about doing the first 2 chapters.
Chapter 1: The survivors of a plane carrying young British boys land on a deserted island. The boys have conflict when trying to elect a leader for their group but ultimately decide for Ralph.
Chapter 2: Piggy, who has just returned from exploring the island, claims that he saw a strange beast on the island. This strikes fear into the younger boys. Ralph has decided that the boys most work together to start a fire for warmth and for potential rescue from planes or ships who see the smoke. The fire ends up going out but the boys still have hope.
