If you open the cylinder you will see a stamping on the yolk. (the bridge that connects the frame and cylinder) It will have a model number and it might have a dash with a series number. Obviously if it is a series 1 gun it won't have a dash with a second number.

There could be a stamping on the top strap also. That's the piece between the hammer and barrel above the cylinder.

It looks like a model 10 series or equivalent by specifics it could be a 10,11,12,13 or a newer 64.

If the top strap is stamped M13 it is an Air Force survival pistol which were all recalled...don't shoot this one.

The only 3"bbl that Smith produced like that in any numbers was the Model 13 Military/Police. Obviously it could be a cut down 4" of any of the other models.