To be honest, a three inch spread at forty feet range is pretty good, certainly tight enough to be a hit on a human-sized target when aiming at the centre of the body mass. And let's be honest, forty feet is a long way, waaaay further than most pistol engagements with either real weapons, or airsoft ones, typically occur. To put that in context for [SIZE=11.666666030883789px]anyone[/SIZE] [SIZE=11.666666030883789px]trying[/SIZE] to picture that range and how it equates to the average range of a fight, forty feet, is ten feet longer distance than the entire external length of a London Routemaster double-decker bus, [SIZE=11.666666030883789px]and[/SIZE] if you've ever sat at the back of such a bus on its top deck, then you know how people sat at the front appear to you in terms of target size, and that's about 27 feet away.
Moreover, I'd be willing to bet that if you clamped the pistol stationary and fired it, you'd find that it was hand movement which was causing most of the variance in grouping, and there is little anyone can do about that, it's just a fact pistols - both good and bad - are tougher to aim because of their short length, and what you've got, is a good pistol, so as the old joke goes, what you may need to modify, is 'the nut behind the trigger'.
You may be interested to know that when it comes to pistol types for police forces, the 'rule of three' is often employed, i.e. statistically, most pistol gunfights last three seconds, where three shots are fired, at a range of three yards. So the really important thing with a pistol, is getting it out quickly, bringing it onto the target quickly, and firing it quickly. These are things you can practice in your house (dry firing), and that is actually a good place to do it too, because you can turn the lights off and practice that drill purely by muscle memory rather than sight, so it really drills in there. What you may find therefore, is that more useful upgrades for your pistol, would be a laser, a holster you can get it out of quickly and easily, and plenty of practice at drawing and firing (that is unless you are going pistols only, and even then, you don't want to be fumbling about holstering pistols when changing mags or lobbing grenades).