RE: jtec/radiowave Extra 300
I personally don't think that you need a third servo on the aileron of a 40% bird anymore. As you say, with the 8611a or 5955 servos at 320oz @6v two is plenty.
Ty seems to feel that the planes with three aileron servos and two elevator servos lock in better, especially in windy conditions. I don't see it yet - maybe I'm wrong. I do believe that with two servos on each surface you are minimizing the play around center you get with one servo. So maybe the set up to go with is two on each elevator and two on each aileron. On my new Extra, however, I am going with two 8611a on each aileron and a single 8611a on each elevator. Hoping to go with two 8611a on the rudder, push-pull in the tail.
Something else to consider is that an extra servo on each aileron, each elevator and the rudder + two extra matchboxes (for us JR guys) linkages, horns and servo leads probably adds up to close to a pound.