LR3 does not support the D750s version of the NEF raw file format.
To import your images in LR3-
GOOD: Use Nikon software to create TIF files from your NEFs. (markedly increases your file storage! more than double)
BETTER: Use the (free) "Adobe DNG Converter" to convert raw NEFs to raw DNGs. (Has some advantages)
BEST: Upgrade to LR6,
it says im updated.
You probably are up to date as far as Lightroom 3 is concerned.
However your camera was not supported until Lightroom 5.7
Support for new cameras is never added retrospectively to older versions of Lightroom.
So often in the digital world, new camera = new (or updated) software.
As mentioned before, your options from an Adobe software perspective are: upgrade to Lightroom 6 or join the Cloud or use the free Adobe DNG converter