Thumbnail sketches are small drawings which help plan your larger, finished, works. There's noting 'finished' about them, and they are generally thrown away once you begin the final piece. So why not use 3D software to help generate some ideas? As far as 3D software goes, Sketchup is very basic and straightforward. Even if you aren't interested in making complex models in 3D, doing a quick block-in of your scene and then moving around it in 3D space can help you select dramatic camera angles.