How have you created this 3D object? In CInema 4D? Element 3D? Something else?
Whatever the answer to that question is, the answer to your original question is the same: Just rotate the 3D object.
well.. what am i doing wrong? what tool do i have to use to just rotate the 3D object and not the whole scene?
Are you animating the rotation property of your 3D object? Hands off the camera.
In Element, if you only have one object, rotate the emitter.
Although, you could rotate the camera and leave your background as a 2d layer.
that's what i did. but i think i need the background layer to be 3D to use the light-layer, right?
You can't use the camera tool to rotate. You have to use the effect controls in Element.
Effect controls will not change any other layer in AE.
If the background is set up in Element, you will need to have it on a different group from your object.
If the background layer is just an AE layer, it does need to be 3d in order for lights to effect it, but if it's not moving, you can create the background other ways without making it 3d. However, you should be able to leave it as a 3d layer because you're going to be animating the Element 3d effect and not the camera, so it won't move anyway.
That makes sence, thank so much!