In 3D Parallax, your project is made up of layers.
By selecting the main object, you create a separate "layer" for the subject of focus, which separates it from the original image. Each element and effect that you add creates a new layer. Having separate layers for each element gives depth to the image and emphasizes the 3D effect.
In the example below, each element has its own layer marked and named accordingly.
Now, a layer's position on the timeline is very important.
Remember: Moving a layer to the left of the timeline puts it "closer to the camera" and moving it to the right places it farther away.
If you are working with several elements and you want them to appear on about the same "plane" you may drag and position them to be on top of or very close to each other on the timeline.
Check out the demo below to see how the position of the layers on the timeline affects how they appear when the camera tool is applied.