I've smoked to many ribs to remember and here's one thing I can tell you from dozens, and dozens of racks of experience. St. Louis ribs are more like 2-2-1. The 3-2-1 is a bit long and better for fattier baby backs IMHO.
When I smoke my ribs using the 2-2-1 method I smoke them around 225-240 and 2-2-1 has always worked on St. Louis style ribs.
Both on a offset and a electric smoker.
Good luck and keep us updated.