it is important to understand that for this trophy you need to initiate blade mode, and cut 3 enemies in one slash.
This isn't merely slicing up 3 enemies during blade mode - you need to chop up all 3 in one cut.
You can easily get this during the first mission (Coup d'Etat). There are enough enemies around, and if you lure a group of standard enemies to you, you can simply initiate blade mode, take a swing and have 3 dismembered bodies at your feet.
Questions, correction and neg vote justifications below.