Chiron's ultimate (
or 4) transforms him into a constellation, and he can fire three long-distance shots, even through walls. In order to unlock this achievement you must kill three opponents in the form of a triple kill in one use of your ultimate. Finding three enemies with low enough health and grouped in a position to kill them all would take a lot of planning and luck so it may be best to boost this one out of the gate.
A fully maxed out Chrion can do in the realm of 400-600 a shot depending on his target. You are literally looking for people on extremely low health, or catching people working on an objective (fire giant, gold fury, siege minion) as most times people are unaware of their surroundings and the impending doom.