I have used 3 springs before... For me this was only because the track i was at was a real loose track , I also used secondary springs and weights in conjunction with this.. Some builders i have heard will do something similar to this and add a TON of weight to the arms .
IMO this is a very crude way of making the clutch act like a two stage clutch, however i have seen it work with ok results. I have seen this used to get a smaller motor to pull big tires, however i believe there are other ways to do this.
Lately i have been using a modified hub/pressure plate to measure total spring pressure.
chris