So I have all my resources scaled up to @3x, but I don't know what size to scale up my background images to. I currently have them at 640x1136 for @2x, but this would mean 960x1704 for @3x which is going to look pixelated on a 1080x1920 device. What do I do for these background images?
Just like iOS 7 assign @2x image, iOS 8 will assign @3x image. To achieve these you have to add app icon, launch images & background images in image Asset.(creating new image asset..) so iOS will automatically assign appropriate image related to device or iOS.
check these links for more details:
http://aten.co/2014/09/12/designing-from-iphone-5-to-iphone-6-iphone-6-more-than-i-bargained-for/
Hope this will help you..