The issue I see with using 3x on the rumoued 12~ inch iPad is there's no improvement in screen real estate, which should be the whole point of a bigger display.
It may be decent enough, but that doesn't mean Apple won't want to improve it even further. They could render at 3072x2304 and downscale to bring the iPad Air to 326 PPI like the Mini. Or maybe they'll just go all the way and bring it up to 396 PPI. We'll see soon enough.
Maybe my math is off. I'll have to redo my calcs.
EDIT:
Steelbom was correct to say that 3072x2304 internal at 3x would mean no increase in real estate. Brain fart on my part.
Without actually going through the nitty gritty calculations, I'm thinking it may make more sense to approach this from another direction. You guys tell me if I'm right.
Assuming a near-12" iPad size (not 12.9"), then two possible options I see are:
2560x1920 at 2X
3840x2880 at 3X
What's interesting about this is that 3840x2880 at 11.97" is exactly 401 ppi, which is the pixel density of the iPhone 6 Plus.
Similarly, the 16:9 5.5" iPhone 6 Plus has a screen height of 4.79". A hypothetical 4:3 11.97" MaxiPad would have a screen width of 9.58", which is exactly twice the screen height of the iPhone 6 Plus, which of course makes sense since it's the same ppi and twice the pixels. That means Apple could use the same panel tech for the 12" MaxiPad as they did for the 5.5" iPhone 6 Plus.
OTOH, I still think it's overkill to do this. 264 ppi for a ~12" tablet is perfectly fine. So, I'll stick with 2560x1920 for my prediction.
BTW, as mentioned, either option would mate well with a detachable keyboard. The keyboard would have full-sized keys, and it could also have a decent-sized trackpad. That is impossible with the 9.7" iPads. (It has been done with the 10.6" Surface machines, but that's because those are widescreen, and bigger.)
With such an optional keyboard, this 12" iPad could represent Apple's first iOS hybrid, and maybe this is the new 12" Retina device that rumour sites were predicting. IOW, perhaps the "iPad Pro" and "12-inch Retina MacBook" are actually one and the same.
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P.S. Can a PowerVR GX6650 even drive a 3840x2880 screen well? If not, then 2560x1920 makes a lot more sense from this perspective too.