Uneven backlight + yellow tint

Yellow spots are usually caused by liquid or pressure damage, underneath the LCD Sheild. If the phone was submerged in alcohol, it's common to have yellow spots, or in your case the whole screen, is a yellow tinge. This normally disapates over time...it really does depend on whether on how much static damage was done, from whatever liquid made it in, and where.

If it's worked on the third phone. Did you change something on the screen, itself? And, have you tried back the other screens, after the third phone?

This kind of damage, is usually related to the screen. If it's logic board related...then it's a hand-off to a skilled, micro-circuitry solderer, for work on the digitizer, or to solder a bridge on the backlight fuse.

Robert, since the LCD display works properly in at least one phone, it appears as if the others phones are having some kind of issue with either the backlight coil or IC (most likely the coil) Those are board level repairs and unless you have the tools and the skills to solder very small components, I'd leave it as is. Of course, you did make sure that all of your settings are properly adjusted. Hope this helps, good luck.