I use a DR-3 that I bought on eBay with my D800e. It came with a short tube (maybe the the DK-7 mentioned by others) that adapts the smaller thread of the DR-3 to the larger thread of the D800e and all other round eyepiece viewfinders since at least the F5.
The DR-3 does not have the 2x ability of the DR-5 or DR-6.
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