I am looking at buying a Nikon D810 with 20mm 1.8, 50mm 1.8, and 85mm 1.8. I use LR 5.7.1 and Windows 7.0. I found the files that LR stores its lens profiles in and all three lenses are listed there. My question is: After I upload a picture to LR, will LR automatically find the correct profile from the LR lens profile files and use it for corrections?
No, not automatically. If you select to manually apply the lens correction, it will use the right one automatically - assuming it has the profile for that particular lens.
One option is to create a developer preset that applies the lens correction, and use this preset during the importing process. This way, all the imported photos will have the right lens correction applied, but nothing else.
Let me know if it's not clear enough and I'll try to take some screenshots of my setup ;-)