The 3-2-1 method is tried and tested and is the one I use too. Do not worry about the internal temperature though, for ribs you know when they are done when you get about 1/4 to 1/2 inch pull pack of the meat on the rib bones. You can also use the toothpick test (a toothpick can be pushed through the meat with little to no resistance) or the bend test (lift up the ribs at one end and where the ribs bend the meat starts to crack).
The 3-2-1 method is not absolute though as the ribs are often ready before the full 6 hours. I usually find that I do not need the whole of the last hour.