Hi Pat
There are three areas that could cause the 3rd SCV to be "sticky".
One, as you said, could be rust in the cable itself. The second could be the control lever itself. Three screws, on the tire side of the fender, hold the cup holder in place. Pull the black handle off the lever and lubricate the pivot point of the lever. Third, is the piston that the cable is attached to, at the valve. This is accessed by removing two nuts, at the front, and two bolts, at the rear of the seat frame. The seat and frame come out together (heavy). You can look down at the valve and see the piston. There could be some corrosion on the exposed part of the piston.
Hope this helps.
Scott