The 3 and 7 is not the button, but the number of the note in the scale. The C scale is C D E F G A B, so in counting that, the E is the 3rd, the B is the 7th. I don't know music theory either, I just learned this basic stuff to know what people were talking about.
Pretty sophisticated nuance from a group considered to be "unsophisticated" by most standards. I find this, like so many other revelations of Cajun music, to be tokens of the art.
"Simple music from simple people on simple instruments," was a description given to me. However, it takes years to figure this simple stuff out.