It’s definitely not a 3D Touch issue. If you’re seeing vertical lines, either with a multicolor/kind a green-ish, it’s a faulty digitizer. There was another thread on this exact topic that someone was experiencing with a different iPhone model, but experienced the same symptoms in terms of when you Apply pressure to the display, it gives you seven layered vertical lines that appear on the display.
Another way you can tell, is if you apply pressure to the display aside from vertical lines showing, the ’pinpoint accuracy’ on the phone will be off-center as well, meaning if you try to touch a specific app/icon on your phone, they may not react immediately or have a slight delay where it’s not registering properly.