yeah, they should be the same. There are a few options you can do, replace the gears in the servo's if you think the rest of the servo is fine, replace the servo with an identical probably sub par sero, or outfit the heli with 3 new better quality servo's. The one thing you can't do is replace the single servo with a better servo because it will probably have a different speed and range of motion compared to the other servo's which could really give you a bad day =)

Good luck!