Just a suggestion: before starting the 'final' render with all the renderer parameters pumped up, make a pre-render with very basic settings (displacement mapping, raytracing, shadows and polygon smoothing off, maximum texture resolution = 512, pixel samples 1, postwork filter size 1, no 3d motion blur, no depth of field). Also you want to render to an external window of half the size (or even less) of the final render, like a 640 x 480 for instance. That way your render would take a lot less time, and you'll have the possibility of tweaking those lights again well before 3 hours from your start... Hope this tip helps...