Okay, the stories are used in the novel as a way for the boys to problem solve. Talking about the different stories enables them to imagine what a deserted island will be like. In addition, they serve other purposes as well.
In all three of these stories, the stranded children remain on the "good" side of evil and live without adults.
Coral Island is alluded to more than once and the story has many similarities. The characters have similar names, and they have them on purpose..... that's what Golding wanted. Coral Island's stranded boys remain innocent and civilized. No matter that they're surrounded by savages, they themselves remain who they are.
In the Lord of the Flies, Golding wanted the exact opposite...... he showed us the other side of the coin.