Ribs are more done by time them temp. because you really can't get a good thermometer read on them.
3-2-1 & 2-2-1 are basic guidlines that people adjust to their liking. 3-2-1 is used for spare ribs and 2-2-1 is used for babyback ribs. The first number is number of hours ribs are in the smoker just sitting on the grates and cooking. Second number is the number of hours ribs are then wrapped in foil and put back on the grates and continue to cook. Third number is number of hours ribs are removed from foil and put back onto the grates for saucing and to firm up a tad.
Most people cook ribs with the smoke chamber between 220° & 250°. If you leave the ribs foiled for the full amount of time indicated by 3-2-1 or 2-2-1 there is a good chance they will be almost to tender. Most people adjust the second number till they get a texture they like, I would suggest for your first attempt cut back the second number to 1.5 hrs. in the foil.
Another option is to run the smoker lower (200°-225°), and dont foil the ribs at all. Starting about the second hr. spritz the ribs 1x per hr. till they are done. You will know when they are done when you pick them up about 1/4 of the way from the end of the rack and the rest of the rack hangs straight down - this is known as the bend test.
For your first go I would suggest using 3-2-1 or 2-2-1, depending on rib types. It is the most forgiving and does turn out good tasting ribs.