Find and write down a line in ch. 3 that might serve as a foreshadowing of something dark in Holden’s future. catcher in the rye

The line in chapter 3 that introduces it as a foreshadowing is "shooting people hat."

Explanation:

In chapter 3, Holden, the protagonist of the story, says that the hat he is wearing is a hat to shoot people. This is used to present a harbinger of Holden's future that will become more oppressive, sinister and obscure, like shooting someone.

Foreshadowing is a literary resource that allows a situation that will happen in the future events of a story to be referenced and presented in a subjunctive way, before it happens.

"The Catcher in the Rye" tells the story of Holder, a very downtrodden and depressed boy, who feels disillusioned within the adult world full of controversies and falsehoods.

as for symbolic actions, the narrator's tearing down the wallpaper in an attempt to find the "woman" in the wallpaper represents her struggle to retain or regain her sanity.   the wallpaper has been part of her confinement and by her tearing it down, she is freeing herself from that confinement.