How much is a S&W .357 magnum Model 19-3 in good condition worth?
A lot depends on what you mean by "good condition." On the grading scale, this is not particularly high, but most people who say this mean it in a more general way. So photos are a big help to us when we are determining what the actual condition of the firearm is, and its attendant value.
The Model 19 reached its pinnacle of design with the dash 3 and dash 4 variants. S&W began production of the 19-3 in 1967 with an improvement in the rear sight and the 19-4, which came out in 1977 was a slightly improved variant that had the gas ring relocated to the cylinder. Both of these are very fine revolvers and were better than the post-1982 Model 19s which did not have pinned barrels and recessed charge holes.
Right now, Model 19-3 revolvers in high graded condition are in fairly high demand. Post some photos if you can. Nickel finish revolvers of this type tend to carry a slight value premium over their blued cousins, and barrel length can be a factor too. All original parts make the gun more valuable, as does the presence of the original box and tools.