The Surge 2
57 Achievements
35-40h
Three Birds With One Stone
Kill 3 enemies at once using the homing missile
2.3%
How to unlock the Three Birds With One Stone achievement in The Surge 2 - Definitive Guide
EDIT (26/08/2020): I restarted the story in a new save slot and I unlocked it on spiders in Gideon rocks, probably the first save slot was bugged for me.
If anyone has trouble finding the spot I will make a video for you when I get a chance.
If anyone is curious where to get the missile, it is called "SSM 'Vanguard' Missile" and you get it from the Hunter you meet before fighting the boss in Gideons Rock if you tell him you will split the reward. Once you beat the boss you will find him in Seaside Court, give him his share and he will give you the missile. You will only have one, if you want more you can buy them from Rex after completing his quest chain.
Once you have the module equipped, try to find a large group of lower leveled enemies and activate it with to hopefully kill at least three at once. You can see a good spot for this in the video below. If you mess up, more ammo for this can be purchased from Rex, assuming you have him available as a merchant (save him from the Children of the Spark). The extra ammo is 30k each, so it might just be better to quit and reload your save if you fail.
The SSM 'Vanguard' Missile is the module you will need for this achievement. I unlocked it by first agreeing to split the profits of the hunt at Gideon's Rock with the hunter, then after the world changes you will find them located in Seaside Court. Agree to pay your half of the share (5k scrap) and he will give you the module. You can also purchase it from Rex.
Once you have the module equipped, try to find a large group of lower leveled enemies and activate it with to hopefully kill at least three at once. You can see a good spot for this in the video below. If you mess up, more ammo for this can be purchased from Rex, assuming you have him available as a merchant (save him from the Children of the Spark). The extra ammo is 30k each, so it might just be better to quit and reload your save if you fail.