I am shooting with I Nikon D750 and I am using Lightroom 3. When I try to upload a raw (nef) file, it says preview unavailable, rather than showing me an image preview. I have tried a number of updates and still can't get it to work.
Nikon D750 raw files are not supported in Lightroom 3.
You need Lightroom 5.7 or higher version for that.
So, you can either upgrade to a higher version of Lightroom or download free DNG Converter to convert the raw files to DNG and access those DNG files in Lightroom 3.
Reference article: Cameras supported by Adobe Camera Raw
The D750 must have Lightroom 5.7 or later, to be able to read the raw data from the D750 NEF images. Any upgrades to v3 are not sufficient.
You have two choices-
1. Buy the Update to Lightroom 6.5 or subscribe to Creative Cloud Lr-CC2015.5. (Note: Be aware that LR 6.5 requires a 64bit computer.)
2. Stay with v3 and use the Adobe DNG Converter to change your NEF files to DNG raw files, then import the DNGs.