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.

Greg

All of them die by violence in some way. The 'littlun' with the birthmark mysteriously disappears and is presumed dead; Simon is "sacrificed" in a barbaric way by Jack and his hunters; Piggy is smashed upon the rocks at the beach by a boulder pushed over the cliff.

Piggy, Simon and 'the littlun' also are set apart by some sort of physical 'mark.' For 'the littlun' the mark is evidently the birthmark, a rather insignificant imperfection. Piggy and Simon, on the other hand, have real health problems. Besides being clumsy and heavy, Piggy is nearsighted and asthmatic; Simon suffers from epileptic seizures and at times has hallucinations.

In the economy of his narration, Golding uses these physical weaknesses as an element of foreshadowing. On the boys' island and in their "fallen" world, there is no place for the weak or for the outsider, and nowhere safe to go.  Most probably, Ralph would have also met a violent end had not the boys been found and rescued in time.

In Lord of the Flies, what do Piggy, Simon, and the littlun with the birthmark share?

Piggy, Simon, and the littlun with the birthmark are the three characters that represent rationalism and civilization.  Those three characters do not descend into the depths of depravity that the other boys like Jack go down to.  Simon and Piggy are quick to attempt to maintain an adult like civility about themselves, other boys, and their "society" on the island.  Piggy frequently quotes his aunt which is interesting, because it serves as an adult voice and as a voice of a female.  Piggy represents a strong voice of reason.  Simon as well is a voice of reason, but he is more sacrificial in his actions.  He has a watchful and caring heart and takes care of the boys that are younger and unable to take care of themselves.  None of those characteristics are present in Jack, who is abusive, aggressive, selfish, and doesn't think through his actions and consequences like the other boys.