What is a 3-bolt rear axle/rear end?

3-bolt rear axle assemblies have 3 bolts attaching the axle to the wheel hub. 4-bolt axles have 4 bolts, as well as beefier axle cups, and are therefore far stronger. See Tom Stangl's 3-bolt to 4-bolt conversion VFAQ for lots of great information. You can also read the answer to How can I tell if my car has a 4-bolt rear end? in this FAQ.