3-2-1 is just a starting point. The truth is, you can foil at anytime during the cook. It all depends on how much smoke you want.
Think of foiling as a time saver. All foil does is steam (or boil) the ribs. It blocks smoke from getting to the ribs but it does speed up the cooking process. That's the only purpose it serves as far as I'm concerned. It allows you to bring a bit more predictability to the cook time. If you are not in a hurry, foil isn't necessary.
Ribs don't need foil as long as you keep the temps at 275 F or below. But, if you do foil them, make sure you cook them a while after foiling to dry them a little. That gets tricky because you need to get them out of the foil before they fall apart and let the finish cook time bring to the tenderness that ribs should have.
Just understand the purpose of foiling ribs. It's only to speed up the cooking process. I have cooked great ribs without foil and prefer to cook them that way. But, if you can get them out of the foil at the right time and cook them a little more, they will still be exceptional.
