The Bullet Train achievement in Hitman 3 requires you to take out a guard with a silenced pistol, a shotgun, and an SMG. This is an easy achievement to unlock and can be completed during the game's final mission, Untouchable. You’ll need to shoot the same guard with all three weapons to get the achievement to unlock.
Throughout the mission, you’ll be taking out guards armed with pistols. Make sure you pick one of these up. About halfway through the level in one of the train cars, you’ll come across a workbench. Here you can craft a silenced pistol. Once crafted, carry on making your way through the train.
After passing through a few more train carts, you’ll come across an armoury area with two guards in it. Kill the guards and pick up the silenced shotgun and silenced SMG on the table, if you haven’t already picked them up from elsewhere. Keep moving forward and exit the train cart to the outside. Jump up and over the container, and you’ll see a guard crouching down working on something. Shoot him once in the leg with the silenced pistol, again with the silenced SMG, and then finish him off with the silenced shotgun.
The achievement is now yours.