Why do my 3D layers get repositioned when I add a camera?

You can think of AE as using a "Default" camera in your composition even when you don't have a camera layer. My suggestion would be to try and match this default camera with your camera layer.

Select the camera and press cmd-shift-y (mac) or ctrl-shift-y (pc) to get to the detailed camera properties. Just under the "Name" on the upper right side there should be a "Preset" dropdown. Select "50mm" from that menu (this matches the default AE camera). Press OK in this dialog.

Now in your composition you just need to make sure that the position of your camera is at the default, to do that twirl your camera properties open on the timeline and hit "Reset" just to the right of the "Transform" properties group.

That should do it.

Darby Edelen