3D Camera - Background

If the background is not moving, you can use a 2D layer. It will be visible in the Active Camera view. 

If you need parallax shifts, so it is like your camera is moving around in the real world, then you can Park a 2 or 3 times comp-sized layer back in Z space, and it will react the same way the horizon does in the real world. If you need to orbit all around the center layers and have a whole world in the background, you have a couple of options. The first is to use the C4D render engine, which limits the 3D layers to no effects, no track mattes, and limited shadow behaviors (Check the Comp Panel for all the restrictions.) This allows you to set a 3D layer as an environment layer, and it will show up in reflections on other layers. You can use this technique to fake a horizon by Parenting a reflective layer to the camera. Trapcode Horizon also creates infinite horizons, but you must purchase it from Maxon. It is part of the Trapcode Suite. 

If you could better explain your design goals or point to a sample image or video, it would help us point you in the right direction.