For this achievement I have easy two methods so I will make two tutorials in one.

[ Register or Signin to view external links. ]

Nazi Zombie Method

1) Load up the zombie map "Nacht der Untoten".
2) Buy the KAR-98 from the wall.
3) Let all of the zombies in the first round get into the room.
4) Avoid the zombies until they are following you in a group.
5) Head towards the top corner inbetween the window and the door to the question box.
6) Wait in the corner until the zombies are walking in a line in front of you, towards you.
7) Aim at the first zombies chest and line him up with the zombies behind.
8) Shoot and if you killed three zombies, the achievement will pop up.
9) If you didn't kill three there should be enough zombies left to try again!

Vendetta Method

1) Load the mission "Vendetta".
2) Play through the mission until Reznov first stands up, jumps over the wall and runs to the car.
3) Follow Reznov to the car.
4) There will be a lot of German soliders and a tank coming right in front of you. Reznov tells you not to kill them.
5) Just wait till three soliders line up and shoot them.
6) You will die but get the achievement.

Here is a video tutorial: