Might just be cheaper and better to let S&W put a new 3inch barrel on it for you. You could then sell the old 6inch tube for a few bucks. And then you would know it was done right and resell value would be higher.
With the shorter barrel, you may need to change out the ejector rod assembly also so, you may be dealing with more than just a barrel swap which costs around $80 plus the part unless you're an experience person with the correct tools for the job. At that point, your better off selling the 6" gun and look to purchase a 3".
I have been looking for one for some time. Finding one is not an easy task. I have not seen a 686 thru 686-5 with a 3" barrel for sale. Yes the newer ones do show up, but I won't own one with the ILS. The gunsmith that's doing the work told me you don't have to change the rod assembly until you go down to 2 1/2".
Jim Stroh/Alpha Precision in Comer,GA has shortened three S&W barrels for me, he is working on one now. Jim is a craftsman of the highest order, your barrel will look like a factory barrel in every respect. You cna contact Jim at (706) 783-2131 or [email protected]. give Jim a call, he is a real gentleman, member of the American Pistolsmith's Guild too!