About halfway through the "Heroes End" chapter you will encounter a major story line cut scene event involving a helicopter and a rocket launcher. After this event pick up the rocket launcher and through the next parts of the game you will find many groups of soldiers to blast.
There is also another part shortly after this where you find another rocket launcher and then have to defend your position. This is also another great point to unlock this achievement.
The blast radius on the rockets is fairly large so you won't have much trouble killing 3 soldiers at once.