What do Piggy, Simon, and the littlun with the birthmark have in common?
These three boys' deaths also serve to form the link the shows the progression of the degeneration of society. First, the littlun is killed more by accident and lack of survival sense. His death, in the eyes of these boys, would have been more minor because he was so young. Simon's death occurred in a frenzy that was instigated by several factors at the time. His death would have been a bigger deal because he was a little older (stuck between a bigun and littlun) and slightly more visible to the other boys, although his often excursions into his creeper place kept him away from their society. Piggy's death would have been worse because he was as old as Jack/Ralph and just as visible. His death certainly would have been noticed by everyone on the island. Of course, this link would have probably continued until its finale, the killing of alpha-dog Ralph, if the the naval officer hadn't arrived.
Piggy, Simon, and that "littlun" share one thing; they are all outside the boundaries of the new society coming into being on the island, and they are outsiders in part because they see things differently. Piggy is an outsider because of his weight and his asthma, and his literal different eyesight (and his personality). His glasses let the boys start the first fire. The marked littlun is an outsider because he sees the snake and insists it exists. Simon functions like a religious prophet, seeing things with special spiritual clarity.
Piggy, Simon and the littlun with the birthmark have one thing in common, which is that they are all different from the majority of the group in different ways. Piggy doesn't have the 'fair' type of body that others do, and he also has asthma. The littlun with the birthmark is always insisting that there really is a creature (snake). He was pushed by the other littluns since others didn't fully believe him and he was the one that started the rumor. Lastly, Simon's not physically different but mentally. He is "always throwing a faint" and he is the newest addition to the choir boys. Simon is the mystic which later on understands the true nature of the beast.
Also, at the end of the story Piggy and Simon die and the littlun goes missing and is presumed dead.