That's bizarre. I haven't noticed yellowing on my Wii U or 3DS screen, but I do play those systems fairly often. I'm sort of afraid to look at my Switch since it's docked pretty much 98% of the time. The only other handheld of note that I have is a PSP system I found at a thrift store last year, and while the screen is pretty scratched, I haven't noticed any weird discoloring. Do you live in a really humid / non-humid area? Are the system's screen exposed to sunlight often?
Edit:
Those screens, especially the DSi one, are egregiously discolored. Even after looking at some of my older handhelds like the GBA SPs, and the original Silver DS system I have, none of them even exhibit the slightest yellowing, but the SP systems do have a weird issue where it's brighter in the middle and darker around the edges, likely due to the "front lighting" the systems have. I might try to take some pictures and see if it's something a camera picks up better / differently than the human eye, but I imagine if my handhelds screens were as yellow as your systems are in person, it'd be almost too much to overlook.
If it is environmental, I live in high altitude, so I don't ever see disc rot or corroded / rusted contact pins since there is simply almost no humidity here, but it seems console shells are much more susceptible to UV light and tend to yellow and discolor quickly. I wonder if that has anything to do with these screen issues