The birthmark faced boy is the first. He is accidentally caught in the first fire that burns out of control on the mountain top. Although his death is accidental, the boys choose to ignore it. Because he is a just a littlun, his death is ignored by the other boys.
The second is Simon. He's a product of a sever fear of a beast in the woods and very bad timing. He was unfortunate enough to stumble out of the woods with good news at a time when the boys were ready to kill something. He is surrounded by the group of chanting, dancing boys and beaten to death on the beach.
Finally, Piggy is a casualty of negotiation. While attempting to retrieve his glasses, Roger drops a large boulder on him which bashes in his head and knocks him into the ocean.