How I did it, it wasn't really legit, though I did get it legitimately afterwards.
Just buy 3 smoke bombs from a blacksmith, and attract 3 groups of guards that are standing or patrolling. An easier way would just to run around while being notorious.
Once you've got them all around in the same general area, throw a smoke bomb and assassinate as many guards as you can while they are coughing. When the first smoke bomb has no effect, throw a second one and continue killing them one by one. The trophy should pop up a few seconds after you get the guards.