Foil is not required, but is a perfectly legitimate and useful tool.
3-2-1 was based on really low temps of 225 or so and St Louis trimmed spares. At 250-275 (a nice temp range), full spares may take just as long.
if SL trimmed or small ribs, less time.
Just watch the color of the ribs. When they are a color you like, foil them with some liquid and any additional flavors you want.
You may, or may not, see a pull back on the meat.
At 1.5 hours or so--see how they bend in the foil.
If they are not stiff and show some bending, open the top of the foil and check with a toothpick or other sharp probe. Or lift them for a better bend test.
When they are fully tender, sauce if you want (I don't) and put them on long enough for the sauce to set up.
YOU CAN NOT COOK RIBS BASED ON TIME--too many variables.
Watch and feel the meat and it will tell you what it is doing.
"Flirtin' with Disaster" BBQ Team (RETIRED)
FBA and KCBS Cook and Judge.
Former owner of a WSM, a Smokey Joe, a Charbroil Commercial gasser (junk), the legendary "StudeDera", a FEC100, a Fast Eddy PG500, and Sherman the Wonder Trailer.
Now cooking with a Yoder YS640
Proud Pellet guy cooking on real wood.