To my knowledge, the
Edge III is terrible quality, and refuses to stay in tune properly.
Edge Pro is the way to go.
At least if you look around, pretty much everyone with an Edge III wants to replace it with an Edge Pro; and as a friend said: "The Edge III is the last in the Edge number series and it was released with about as much quality control as a mexican sock factory".
Feel free to hit me if I'm wrong, but I'm merely reiterating what's already been said.