This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / A Nightmare on Elm Street 3: Dream Warriors
The "flesh marionette" scene.
Made worse with the sight of an enlarged Freddy controlling Phillip from the sky. Also dips into Tear Jerker territory, as the rest of the teens are forced to watch Phillip fall to his death from far away, and scream out his name, clearly worried about him.
The scene where Kristen snaps out of her dream with the killer bathroom fixtures, and realizes she's got a bleeding wrist and a bloody razor blade in her hand. Maybe it's just paranoia talking, but for a split second she's so disoriented, she may not have been sure she hadn't genuinely gone crazy and slashed her own wrist while hallucinating. At least you can fight Freddy, but your own insanity...
The concept introduced that Freddy absorbs the souls of his victims. When Jason, Chucky or Michael Myers kill you, the pain is over. When Freddy kills you, it's just beginning.
The former patient who cut off his own eyelids. Becomes Fridge Horror when you remember the scene that got Kristen committed, and that nobody ever found out how the kid got hold of a blade to do it...
Nancy keeping vigil over a comatose Joey; "Let him go you bastard!" Freddy's reply - COME AND GET HIM BITCH - is carved into Joey's chest for her to see.
The "Freddy wyrm" scene.
"Let's get high."
Poor Joey. He is a mute and cannot scream for help.
The opening sequence avoided retreading what the first two films had done by making it clear at the start that Freddy is a very real and immediate threat, and this isn't a Freddy who was just beginning another killing spree. This was Freddy after he'd already killed and tormented several teenagers, and we're not even told just how long this had been going on or when it even started.
This was the first film in which Freddy's kills became character-specific, preying on their own weaknesses and insecurities. While even some of those deaths are still kind of narmy, it's handled a thousand time better than in later films.
Nancy's death. The audience knows it's coming, but it's still terrifying because she's hugging her father, letting her guard down for a split-second... and then he gets her.