Here's a guide for anyone that needs help with this trophy.
First, load up Nazi zombies, Nacht der Untoten. Open the help door and buy the double Barrled shotgun. Let 3 zombies go to a window and tear it down. When the 3 zombies line up to get in, shoot them with the shotgun once. All 3 should die, and the trophy is unlocked. Now, have fun playing Zombies if you want to continue playing!