On the left side of the screen we can see the properties of every object we choose, you need to select and edit the next parameters on the parallax effect for each layer, you can do this after or before to add the backgrounds it does not matter the order.
In order to have a cool effect, you need to set the background color of your images with no color or transparent when you are editing it, in the files I gave you, you can see it.
Also in the properties panel, you need to change the "Transparent parameter" to YES, just for layers 1 and 2.
Other wise you will never see the background images on diferente layers.
Feel free to change the parallax values as you wish, for see a live view of it, just click on Editor Properties that is at the bottom of the Layer properties section, and choose yes for the Parallax in editor option for our 3 BG layers.
Then scroll the horizontal bar you will see something like this:
As you move the horizontal bar, the background images moves in a different speed, for complete the full effect with our images, we need to adjust the BG2 an BG3 images to a 6000 width. As it is a tiled Background it will repeat infinitely.
NOTE: When we try to add an image or graphic wider than 1024 pixels, Construct 2 will send us an advice of web browser performance, so I suggest to you, be very carefull when you design your graphics.