Word's numbering issues are legend, and can be time-consuming and frustrating, a you have discovered. The jumping is a reuslt of Word automatically reverting to a background numbering format in preset list styles, one of the things that allows automated numbering a paragraph styles. A number of these are built in and often work well; but not in applications such as yours. Without writing a custom macro that would somehow limit your numbering within a style to a,b,c and then restart it (out of my league), the simplest work-arounds I can think of, are

1) manually numbering, i.e., not using auto-numbering at all
2) try manually numbering every other grouping of a-c
3) creating a second, identical list style to alternate each time you get to c.

Measures 2 and 3 may not even work that well, and will require careful checking (rather, unchecking) of all of the possible auto-numbering options in Word's set-up options.