He's Gonna Need a Sturgeon Trophy in Uncharted 3: Drake's Deception

This trophy is earned in Chapter 10 - Historical Research.

During this Chapter you will engage in a chase through the streets and various buildings in pursuit of Talbot. He will escape and you will be ambushed by waves of unarmed enemies in the middle of an open air market. This scripted brawl will force you to melee kill all of your attackers, including a Brute and without the luxury of a weapon. One thing you do have in your arsenal however are the giant fish on the stalls around you which can kill an enemy in one hit if you pick one up and hit them with it.

There is no guaranteed method for this, as you just have to be near a fish counter with an encroaching enemy. If you hit when they are close enough, you have a chance of picking up one of these huge fish and whacking them across the face with it. If not then you will just throw a punch, in which case try again and stay near the fish counters. The Brute can not be killed this way so deal with him the usual way and only attempt this with the normal enemies. You should have no trouble getting 3 kills this way (especially in 2 playthroughs) but in case you do, use the save/reload method to record any successful kills and stack them until the trophy unlocks.

Here's a video guide courtesy of PowerPyx: