The process is quite similar to yours, Richard. I almost never start with a particular concept in mind. In most cases I simply explore and "play" until something catches my eye. More here: [link] .
In the case of this image, of course, I did have a concept in mind - one that started with the Then There Were Three image. It appealed to me enough that I consciously went looking for similar "two" and "one" structures with which to work.