I have a comp with multiple layers arranged in 3d space. A camera parented to 3x Nulls for x, y z respectively.
Where should I apply the wiggle to get a realistic movement when I use the camera ?
No, but if you are using sliders to control the null position, the wiggle expression would have to go to the sliders instead.
You may want to add sliders that control the wiggle amount and speed on a separate control layer (such as a small solid) and link them up to the wiggle expressions with the pickwhip so that you can change the wiggle settings in one place for all sliders.