Formula for sight adjustment
You're in luck. The gun is shooting low. What you need is a lower front sight. It's easy to take metal off a front sight, but hard to put metal on!
As others have pointed out, before you start whittling on your gun, make darn sure it's the gun, and not the ammo or the shooter. You've ruled out the ammo. The only other thing to check is trigger overtravel. Some guns, after the sear releases the hammer, the trigger travels a long way before it finally stops moving backwards. In the millisecond between the time the sear releases and the trigger reaches the end of its travel, the gun can move down a bit. Your S&W may have excessive overtravel, while your other guns may not.
You can check for overtravel by unloading the gun 9 times, putting all your ammo in a hermetically sealed mayonnaise jar on Funk & Wagnall's front porch, and pointing the gun in a safe direction - something that will stop a bullet and won't be too expensive to replace, if you do have a surprise bang. Then, insert various small objects between the rear of the trigger and the pistol frame, and pulling the trigger extremely slowly and carefully. A paper clip, a toothpick, etc. Try to see if the S&W trigger moves a long way. If you can fit a #2 pencil between the trigger and the frame, and the sear will still release, there's too much overtravel! Compare with your other guns.
Target shooters use adjustable stop screws to eliminate all overtravel. Not a good idea on a SD gun; you don't want zero overtravel. You want to leave a little, to be sure the gun will fire if you need it to. If you think overtravel might be the problem, glue a temporary overtravel stop behind the trigger, and go to the range. A glob of silicone glue, or even wrapping the trigger with a rubber band, will work as a temporary stop. IF you are sure that overtravel is the problem, put a drop of JB Weld on the back of your trigger, let it harden, and then file it down until the gun will fire, but won't have a lot of travel after the sear releases. Or, tap for a small screw, use Lok-Tite, and face off the screw until you have the desired amount of overtravel.
If you are positive the sights are misaligned, then you need to remove some metal from the front sight. Here's the formula:
(S x H) / D x 36
S = sight radius, the distance between the front sight & the rear sight
H = inches you want your group to move (in your case, 3 or 4 inches)
D = distance from front sight to target, in yards (hence the 36 to convert to inches)
So, if your front sight and rear sight are 4.5" apart (wild guess; measure with a ruler); and you want to move the group 3", and you are 10 yards away, then (4.5 x 3) / (10 x 36) = .0375. That is, you want to take about 3 or 4 hundredths of an inch off your front sight.
You can do that freehand with a file, or you can build a jig. I find a smooth flat surface (Formica countertop? Smooth kitchen cutting board?) and lay a piece of emery paper on the flat surface. Take the slide off the pistol. Measure the height of the front sight with your dial or digital caliper. Lay the slide down, sights down, with the rear sight on the smooth surface, and the front sight on the emery paper. Slide the slide back and forth so that the emery paper removes metal from the front sight. Check frequently with your caliper so you don't remove too much metal. Repeat until the right amount of metal is removed.
Or, take a very (very!) fine file to the range. Shoot, whittle, shoot again, whittle a little more, shoot, whittle, repeat until the group is where you want it. When you get home from the range, square and level everything up, and touch up with cold blue.
BTW, it's fairly common for guns to be sighted a little low from the factory. The manufacturer can only guess about people's eyes, grips, ammo. It's better for them to make guns that shoot too low instead of too high, because too low is easy to fix.
Just more wood to throw into the fire...
What if it's not a front/rear sight problem, but the way you hold the gun? Just throwing it out there.