I have Lightroom 3 and can't download pictures from my new Nikon D810, I have tried to do the updates but as soon as I open the downloaded update it disappears from my computer screen. I have a Mac computer.
There is no update for Lightroom 3 that will support that camera. You need at least Lightroom 5.6. If you want to stay with Lightroom 3 you can download DNG converter 9.0 and create DNG copies of your raw files.