For this hidden trophy you have to get a single kill with each of the game's three Knives. You must kill either one guard, target or a civilian with the Scalpel, Combat Knife and the Kitchen Knife. You start off with the Kitchen Knife; the Combat knife is found on the General in Mission 4 and the Scalpel is found in Mission 18 - see All Melee Weapons Collected for a guide on their exact locations. To use a Knife you have to equip it and press to attack, it takes a couple of swipes to get the kill. It is incredibly clunky to use and if you are running after your victim it is virtually impossible to run and attack. The best plan of attack is to sneak up to someone and then attack them from behind as you would do if you were using the Fibre Wire.
Important notes for you to bear in mind when going for this trophy:
1: You do not have to get the three kills all in one mission. If you do get a kill, say you killed someone with a Combat Knife, then you must complete the actual mission for it to be recorded. If you just quit or die it will not count.
2: You can obtain the Scalpel in Mission 18: Terminal Hospitality. The Scalpel can actually aid you in killing this mission's target, however, he will not count as one of your kills so you have do this somewhere else.
Kitchen, Combat Knife and Scalpel Guide
In your first playthrough do not fret too much about this trophy. You can easily factor these kills in without adding any unnecessary time to your clean-up playthrough. I briefly touched on this in the Roadmap but the best place to get these kills is when you are redoing Mission 1: Anathema. You have to redo this mission twice anyway for the Golf Club and the Shotgun. At the start of the level there is a guard who goes to the toilet and then you have the mission's target. This presents you with two opportunistic kills per playthrough, make sure you equip all three Knives and sneak in those kills whilst getting the two weapons.