user8018: Camera
[comp 3D]
  ellipse mask in z-axis
  layer1 2D
  layer2 2D
  layerN 2D

The layers are aligned in the Z-axis along the ellipse, forming a 3D text by the layers.
When I'm inside the composition, I can actually see a aligned layers in the 3D space.
But when I go to my main project view just seing the "comp 3D" layer, everything is plain 2D! Also moving the camera I can just see a plain 2D image, even though inside the comp it is 3D. Why?
Update:

There are 3 lights being aligned differently in the z-axis (see TOP view).
When I move a camera around inside the "light comp", everything is fine and I can see also my lights moving.
When I move the camera in my main layout, the lights are static! No movement is reflected to the light comp layer! I can just see the camera mask moving, not the comp content.

user3295: The issue could be Optical Flares, it doesn't fully integrate into AE's 3D space but has its own coordinate system. The usage of AE lights is somewhat of an workaround for that.
Have a look at this plugin/script, that could help:
http://www.videocopilot.net/tutorials/3d_pre-compose_script/
It's exactly made for what you want to do.

user4426: You need to click the Collapse Transformations (sun icon) on your [Comp 3D] layer.
Otherwise your main comp will treat your other comp as a flat layer in a 3D space, if that makes sense?


8018: <pre><code>Camera
[comp 3D]
  ellipse mask in z-axis
  layer1 2D
  layer2 2D
  layerN 2D
</code></pre>

<p>The layers are aligned in the Z-axis along the ellipse, forming a 3D text by the layers.
When I'm inside the composition, I can actually see a aligned layers in the 3D space.</p>

<p>But when I go to my main project view just seing the "comp 3D" layer, everything is plain 2D! Also moving the camera I can just see a plain 2D image, even though inside the comp it is 3D. Why?</p>

<p>Update:
<img src="https://i.stack.imgur.com/VTGGe.png" alt="enter image description here"></p>

<p>There are 3 lights being aligned differently in the z-axis (see TOP view).</p>

<p>When I move a camera around inside the "light comp", everything is fine and I can see also my lights moving.</p>

<p>When I move the camera in my main layout, the lights are static! No movement is reflected to the light comp layer! I can just see the camera mask moving, not the comp content.</p>
 3295: <p>The issue could be Optical Flares, it doesn't fully integrate into AE's 3D space but has its own coordinate system. The usage of AE lights is somewhat of an workaround for that.
Have a look at this plugin/script, that could help:
<a href="http://www.videocopilot.net/tutorials/3d_pre-compose_script/" rel="nofollow">http://www.videocopilot.net/tutorials/3d_pre-compose_script/</a></p>

<p>It's exactly made for what you want to do.</p>
 4426: <p>You need to click the Collapse Transformations (sun icon) on your [Comp 3D] layer.
Otherwise your main comp will treat your other comp as a flat layer in a 3D space, if that makes sense?</p>

<p><img src="https://i.stack.imgur.com/fx4fq.png" alt="enter image description here"></p>
