Hi,
What are the document specs (bit depth, size, resolution)? What are you machine specs (CPU speed, core count, RAM)? Have you changed any of the 3D preferences?
If you want to render out the 3D layer for an appearance other than what you see in the interaction mode, then yes you'll need to do this for each 3D layer. There are a few ways to attac this scenario and make it more automatic. Flattening your file will remove any ability to further edit the 3D properties, including the ability to render.
While the interaction and drawing of 3D layers generally require the GPU, rendering still only utilizes the CPU for computation. So if it is just the rendering that is "taking for ever" than the GPU update and troubleshooting won't help. You can interrupt the rendering at any time that you feel the quality is good for your purpose, by any keyboard input or mouse click on the Ps interface (we do have a known problem where this doesn't happen sometimes).
Since you don't have shadows or reflections, my quick guess is either you have transparency some where in the materials and/or the machine spec (low CPU core count, speed) is not strong enough to handle the document spec (high pixel count, 300 ppi resolution, 16 bit mode) in a timely fashion.
regards,
steve
