Typically 3-2-1 is going to be too long for back ribs...I use 3-2-1 for spares.
When I do back ribs (rarely) I do 3 hrs of smoke...1hr in foil...and then maybe 1/2hr back on the smoker.
I have a Weber Smokey Mountain (water smoker) like you have. 3-2-1 is more of a guideline than anything else. If you like really fall off the bone ribs then you want more time in the foil, which would be the 2 part of the equation.
3-hrs on the smoker
2-hrs in foil on the smoker
1-hr back on the smoker without foil (to firm up the ribs)
Hope that helps!!