Doneness on ribs is a personal preference.
3-2-1 is mostly for spares. Some folks use 2-2-1, some 2-1-1, some 2-1.5-1, others 2-1-2.. The combinations go on..... Baby backs are tender so there are a lot of folks that cook them for 4-6 hours without using foil at all.
You should know what is happening in each phase of the process so that you know how to fine tune the ribs on the next cook.
The first step cooks the ribs low-n-slow and blankets them with smoke. It is also where the smoke ring develops. Some folks spritz or mop to keep the exterior surface moist and add a dimension of flavor.
In the second phase braises the ribs and makes them very tender. Flavors can be added to the ribs by adding ingredients inside the foil. Foil also speeds the cooking process and limits exposure to smoke. Knowing this is good for making adjustments to suit your palate.
During the final period they are removed from foil and placed back into the smoker to dry out and firm up a bit. After being in foil the meat can be a little soft and loose, if it is too tender at the point, reduce the amount of time (or reduce the heat) during the next cook. This is the time when you want to glaze, baste, or sauce your ribs if desired, so the flavor can bind to the meat before serving. Some prefer theirs dry.
Hopefully knowing the above will help you decide which times to choose to get the results you desire.