Encounter: Drawbridge (Chapter 10)
Select the Drawbridge encounter from the main menu. You will start out with an AK-47, a pistol and a grenade. There are many enemies close together here.
Just like the trophy description says, you must do the kills in a specific order: Pistol, AK-47, Grenade. Kill 3 enemies very quickly with the different weapons and the trophy will unlock. Very easy.