SDM shortens barrels to 4" and install new sights for $250.
Here's a picture of the process:
4 conversion
SDM SW
I don't know what they would charge to shorten it to 3", or even if they would do it, since they are set up for 4".
However, cutting it to 2 1/2 is a whole different problem.
Suggest you make a close comparison of your gun and a 2 1/2 in 686. Note the ejector rods. Short ejector rods with .357 cause ejection problems.
Although anything is possible, the conversion to 2 1/2 in barrel with new parts probably costs as much as buying a used revolver in the configuration you want. And afer the conversion, the value of the gun would be less. Personally, 3" is as short as I would consider to shoot .357s.