To obtain this trophy, you need to have the Flamethrower skill unlocked in your skill tree (Tech).
Next and important to note, that this only works on enemies that are unshielded. You need to find a group of unshielded enemies together and light three of them on fire with on active Flamethrower attack.
Below is a map showing where I got this trophy. At the red arrow on the map, is a group of approx. 10 Kett enemies.
You can sneak close to this group of Kett if you have Tactical Cloak enabled, if not, just run up on them quickly. They were all grouped together tight enough here that I was able to run up and use Flamethrower on 3-4 close enemies together, setting them all on fire.
Doing so will unlock the trophy.