So, on an Edge III, how many springs would you need for string gauges 10's or 11's?
Well, I always had 3 (set up in an arrow-head position). I'd just tighten/loosen them if I was to change tunings or string gauge.
I had 3 springs when I had 9's on, but I moved up to 12's and had to go with 4 springs. Just go buy an extra spring for a couple of bucks and you'll be set. You could probably get away with 3 springs if you tighten the spring tensions screws all the way, but I don't personally like to do that...