What's more a lot of people (maybe 20%) don't benefit from things like
which is one reason why stereo movies have struggled. (That plus some people get sick... Having both a flat and 3-d movie in two different theaters comes across as money grubbing to the consumer but it is really a money sink to the theater.)
In these later examples (starting with the easy puzzle of the OP, and your 3d examples), I find that I do the process in two stages.
Unfocus my sight until the third image shows up in the middle at the correct size (as a blurry mess). Then try to focus the center image.