Camer Tracking creates an animated camera that matches the camera position and movement of the camera used to shoot the tracked video. If you set an origin and ground plane and then add a 3D null or other 3D layer at the same spot the new element will be at the comp center. If you do not set an origin and ground plane new layers can end up anywhere. I'm guessing that you are just adding a comp and turning the comp 3D or that your comp contains 3D layers and you didn't set up the origin.
There is is a way to move a new 3D layer to the current position of a correctly placed layer. Just reveal the parent column hold down the shift key and parent the new layer to the properly positioned one.
If that doesn't work we need done a lot more about your composition and your workflow.
according to your screengrab, you have nothing parented to the null
Your comp is not at the same position as the null, simple as that. It's also probably not the right size so it needs to be scaled. If there are any 3D layers in your comp you must not have Collapse Transformations turned on. BTW, a screenshot with no properties revealed for the layers you are having problems with isn't very helpful. When you have problems in the future select the layers that you are having problems with and press the U key twice to reveal everything about these layers that is not at the default setting.
To move your comp to the same position as your 3D null simply make the comp a 3D layer, hold down the Shift key and make the null the parent of the null. Problem solved. Note the second paragraph of my first reply.
Note the before and after in this simple example.
Thanks Rick! … I'm new at this, and don't totally understand anchor points, etc.
I also decided that using AE's 3D Tracker might have been too confusing. So now I'm using the data from Mocha, and it works fine with just a simple shape layer. But when I apply Mocha's Corner Pin data to an animated comp, it shifts wildly. Any thoughts?
If you're null ended up in the right position when you did the camera tracking then all you have to do is hold down the shift key when you parent your embedded composition to the null and then scale and position the layer.
I have no idea what's going on with Mocha because you haven't told us about your technique.