Hoohanger,
3-2-1 is a basic guideline for rib cooking. Barebones it means to cook ribs 3 hours, wrap in foil and cook another 2 hours, unwrap and cook another hour.
This basic info applies to spare ribs, for loinbacks(babybacks) times will need to be reduced.
Now for the straight poop. "3" doesn't mean anything. Variations in pit temps, particular cut you're cooking type of smoker you're using etc can really play havoc on this first phase so you have to watch the ribs.
The indicator of when to wrap the ribs is watching for the meat to pull away from the ends of the bones 1/4"-3/8". When this happens its time to wrap 'em up.
Use heavy duty foil for wrapping and use caution not to pierce the foil with the bones. I cook the ribs meat side down while in the foil, and always spray the ribs with some liquid (apple juice is a great place to start) before sealing the foil.
Cook for 2 hours in the foil (a bit less for BB's). A good indicator of when to unwrap is to put on gloves and lift the wrapped ribs in the center, when they're quite flexible and both ends droop it's time to unwrap. To get a feel for this lift freshly wrapped ribs and see that they're still fairly rigid.
Unwrap the ribs and cook another hour. This is when you want to use sauce if you like "wet" ribs. If not using sauce, check the ribs every 15-20 mins (peek quickly to retain heat) and spray or mop when the surface looks dry.
3-2-1 isn't the only way to cook ribs, but its a really good way and an excellent starting point to develop your own technique.