Complex sentence: The three affiliations: "Lancaster", "Holmes Old Order" and "Elkhart-LaGrange" are not only the three largest affiliations, they also represent the mainstream among the Old Order Amish.
Split into simpler sentences: The three affiliations: "Lancaster", "Holmes Old Order" and "Elkhart-LaGrange" are not only the three largest affiliations. They also represent the mainstream among the Old Order Amish.