The Jupiter-3's have a reputation for wide variations in lens quality. Some will focus well on a Leica, some well on a Russian LSM camera, and many just don't focus well at all. It is a real gamble to buy a Jupiter-3 and my own experience is that one has less than an even chance of getting a useable one. This is true of both the M39 and Contax mounts.
The Jupiter-8's seem much better and more consistent in build quality and chances are in your favor of getting quite a sharp, useable lens. Folklore is that the silver ones from the 1950's are the best but these often have petrified lubricant in focus helix which can require cleaning and replacement. There are at least three different mount designs on the J-8; two of these come apart easily for cleaning, the third requires removal of several tiny screws which can be difficult to remove.
I've been happy with the more modern black bodied J-8, a very sharp lens, even wide open. These usually have an aluminum body but I recent received one, manufactured in 2001, that has plastic arpeture and DOF rings.