On the older Edge/Extra, the longest servo arm you can use at the elevators and have them pointed up is 1-1/8". That length is more than enough for excellent 3D. If you have a rudder servo mounted in the tail, the same length arm, 1-1/8", mounted pointing down, is also as much as you will need.

I strongly suggest that you obtain a good set of aluminum servo arms in the appropriate length, along with strong, close fitting connectors that have both excellent range and freedom of motion. I'm partial to Nelson/Rocket City RCL70 stuff (www.nelsonhobby.com). Others like JR or Dubro. Both SWB and Airwild produce excellent servo arms in the lengths you will require. The stock, short Hitec arms won't be able to do the job for 3D no matter what you do.

Pat