You're welcome.
In my opinion all three are good. It comes down to personal preference on your part. The only difference, besides finish, on the 520 & 620 are the stocks and sights, otherwise it's the same gun. The barrel is 2 piece on both of them as opposed to one piece on the 686P. I don't know the benefit of one from the other but folks on this forum that know more than I, say the 2 piece barrel has it's advantages and is no drawback.
Do you prefer a full lug barrel? If so the 686P is your gun. Do you prefer the finish on the 520? If so, it's your gun. Do you a prefer stainless 1/2 lug? If so the 620 is gun. That's how I would look at it anyway.
Good luck!