The link between these three chapters is the way in which the boys become more and more uncivilised and savage, indicated in "Long Faces and Long Hair" by the killing of a pig, then indicated once again in "Gift for the Darkness" by the way that another pig is killed and its head is impaled, and then finally in "Cry of the Hunters" by how Jack and his hunters actually try to hunt down Ralph and kill him, as they have already killed Piggy and Simon. Note how this descent into savagery is described in the first of these chapters, which is Chapter 4:
His mind was crowded with memories; memories of the knowledge that had come to them when they closed in on the struggling pig, knowledge that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink.
Note the joy and excitement Jack feels at having "imposed" his will upon "a living thing," and "taken away its life like a long satisfying drink." This final simile is perhaps most chilling, as it shows that Jack and the hunters liked what they did. The way in which the pig is referred to as a "thing" makes it non-specific, foreshadowing the other human prey that he will kill later on in the novel. This bloodthirsty nature is something that is reinforced in "Gift for the Darkness" when Roger shoves the spear forcefully up the pig's anus that they kill, and then again in the final chapter, when the group of boys led by Jack hunt Ralph as if he were nothing more than a pig himself. Clearly any semblance of normal, civilised humanity has been lost as the boys spend more time on the island away from civilisation, the link between these three chapters is that the boys are shown to become more and more barbaric and bloodthirsty.