When I try and import to Lightroom 3 from a new Nikon D810 camera the preview boxes are grayed out with a note in each that says preview is unavailable for this photo. I shot the photos in raw+fine which is supposed to also record a jpeg image besides the raw file. When I hit the import button it thinks for a while and then just says no photos available to import. My D300s camera when shooting in the same mode did just fine. Anyone that can help please advise.
LR 3 wasn’t updated for new camera support after LR 4 was released. The D810 requires at least LR 5.6. And currently Adobe is selling LR 6.0, which is a bit buggy as an initial release.
Your choices are to upgrade to LR 6 with its more modern OS system requirements, or use the DNG Converter 8.6 or newer, with also somewhat more recent OS minimum version requirements: