You receive a "Head Hunter" Medal for killing three enemies with headshots in TDM (including 3TDM, FFA, and Classic) and Objective games.
However, it's also possible to get these Medals playing Co-op Hunter Arena - but only when playing as a Villain. As such, the most convenient way to get these Medals is whilst performing Hunter boosting for Treasures.
Note that if you headshot the first of the two Heroes into a "downed" state, you will not get credit for the headshot until you kill the other Hero. Once a Hero is in the downed state, you cannot "kill" them; both Heroes must be down for you to get the kills.
Once again: it's only possible to get these Medals in Hunter when playing as a Villain.