Complex sentence: As in the other two flights, Flight C competition begins each summer at the local club level and qualifiers advance to unit and district competitions with three pairs from each district--more, depending upon attendance--qualifying for the North American final.
Split into simpler sentences: As in the other two flights, Flight C competition now begins each summer at the local club level and qualifiers then advance to unit competition. Those qualifiers then advance to the district finals and three pairs from each district--more, depending upon attendance--qualify for the North American final.