You pose an interesting question for which I would say it depends on how you define quality.
First, I'd like to start by stating that I know just enough to make myself dangerous and get myself into trouble!
But I'll try my best!!
In regards to diffraction, there's two separate issues I believe. The affect on perceptive sharpness and ability to capture spatial frequency. We can correct for the lack of perceived sharpness caused by diffraction; but we cannot recover lost detail due to it.
In regards to perceptive sharpness it correlates to the amount of enlargement of the captured Airy Disk. So in that regard all three cameras are bound by the same amount of enlargement. For DX cameras and an 8x12 enlargement viewed at about 18 inches that is said to be around f/11.
As to spatial frequency, if the diffraction spot is the size of two pixels, it's still considered resolvable. It doesn't become limiting until the spot reaches three times the pixel pitch. As such by f/11 there probably isn't much difference in fine detail between the D7100 and D7000. But most likely a bit better than a D90. Of course all this assumes there is small enough detail in the scene to matter.
But in your poser, you suggested down sampling. So in essence you are throwing away any fine detail advantage in the higher pixel density camera, so in essence the result should be about the same. But in that case the significant difference would be in the SNR which was covered in an earlier post by Stan.