My whole life I've been doing stereograms by diverging, but I couldn't get the three images in the post (the pairs would get closer but never fully overlap), so I tried crossing based on your comment. It was way easier than diverging (obviously, since I couldn't do it otherwise), but it took me a few tries, because I think it's actually /too/ easy to cross your eyes compared to diverging - I was way overshooting when I crossed my eyes. The trick was to notice this, and then control the un-crossing until they lined up.