The IvP-18 Tropov Pistol is one of the default starting weapons for Hakha, so you can try and get this while also aiming for  Field Operative, however, you can pick up this weapon as any other character if an enemy drops it (such as a Base Commander). As mentioned in  Bullheaded, this weapon is very similar in appearance to the M4 Semi-Automatic Pistol, but note that the IvP-18 Tropov Pistol has a burst fire mode, a larger magazine, and (crucial to this trophy), a secondary fire mode.

The good news is that the secondary fire on the IVP-18 Tropov Pistol is a semi-automatic mechanism, and is much more accurate. A fully loaded magazine is 18 bullets, so you should have more than enough for 3 headshots. One headshot from this pistol is enough to kill regular enemies on Easy mode (with or without cheats activated). Just be sure not to reload between headshots.

Also note that they do not need to be consecutive headshots. For instance, you can kill the first enemy with a headshot, kill the next enemy with  body shots, then kill the third and fourth enemies with headshots. The trophy should still pop so long as you killed all of them without reloading.

The video guide below demonstrates the trophy being earned on Level 1 of Chapter 3 "New Allies" (where you can pick up the weapon as either Templar or Luger), as well as the differences between the two pistols.

Tripletapper Video Guide

Video credited to sunnygrg on YouTube.