This achievement is a lot easier than it seems. An easy spot to do this is in the Amanai Cave in the Siwa oasis during the “Water Rats” mission, but the same technique can be used anywhere. In the Water Rats cave, there’s a grassy spot to hide right next to a ladder that leads to the lower part of the cave. Crouch here and wait for three separate enemies to approach (you can whistle to attract them over if needed). Incapacitate each one individually with a takedown. Once you have three enemies all down, but still alive (writhing around – they need to all be alive for this to work) you then perform a quick attack, and this should finish all three off with one blow!
If you already have the hidden blade, JuicyUrchin498
pointed out the following: "You can just equip "Unarmed" weapon in gear menu and it doesn't kill with the assassins blade so the first solution still works after getting the blade. I just hid in a bush and knocked them out and killed them later."
You can also kill hostile animals for this achievement. Thanks to Dr Zoidstein
for the following additional info: "I killed four hyenas with one slash while on horseback and got the achievement. I don't know if killing non hostile animals will work or not but hostile ones definitely do."Darkness Momon
has confirmed that non-hostile animals won't work: "Does not work on neutral animals. Just got 5 goats at once and no achievement."If you downvote, please state why in a comment so the solution can be improved.