The idea is that the third graph is launch-aligned too and the reason for a third graph is the scaling. If you created an example for Switch vs. 3DS and Wii U in the USA, the full life of the 3DS and Wii U would probably condense the scaling to a degree where the current finer nuances would get lost, because for a good junk of the first three years the lines are very close together.
On the other hand, a comparison like PS4 vs. 360 in the USA that is entering its final stage, a separate third graph would hardly change anything, so for such an article it would be sufficient to change the total sales graph to one that shows the full length of the 360's lifecycle and stick with only two images in total.
It's something to decide on a case by case basis, but I suppose that most comparisons would look good with only two graphs where total sales shows the full lifecycle of the older console(s). Maybe you post a couple of examples in this thread and see what people think about them.
When I start doing the April 2020 Gap Charts I will go through them and see which ones make the most sense to add a third chart to. I do like the idea, even if it means a little more work. At least at first when it comes to creating the graphs. Once they are created it is easy enough to update them with the latest months figures.