With all the devices, try metering off something even in tone, consistently lit and filling the metering area. For example a shaded plain wall, or even the palm of your hand. See what happens when they are all "looking" at exactly the same thing in the same light.

Differences may be due to different matrix-metering in the various Nikons and incident versus handheld results from the lightmeter?

Edit: And as per the post above, use the same lens when metering with each of the Nikons!