This is Chrions 2nd ability.
This ability requires you to hit an enemy with a basic attack to mark them, then you use the ability to hit them again. The ability will home so you don't have to worry about aiming. The mark at max level will only last 3.5s.
To get the achievement you have to mark three enemies then hit them with the ability before the first mark goes away.
The easiest way to do this is ult and hit three enemies with the ult then use your ability. The ult counts because it is a stronger basic attack that allows you to hit multiple enemies in one shot.
I'm not sure if you can use masterful shot while in your ult, but if you shoot your three arrows the ult will end.