The three Musketeers achievement

[Blanchflower]

OK, I have a problem here. I am playing as the French and have at least 3 Musketeers in my army with a great general. To unlock this achievement, I have to kill a unit with a French Musketeer when two other Musketeers are providing flanking bonuses.

OK, granted I did not kill any unit from another civilization yet BUT I killed barbarians and units belong to city states by providing flanking bonuses as well as a great general bonus. Does this count in unlocking this achievement? Or must I kill a unit from an actual rival civilization and not city states units and barbarians?

Please help! Thanks!

 

[Poopin Thelift]

I've got the achievement from killing a barb, iirc.

 

[Ravellion]

It might be the case that the musketeers need to be adjacent to each other (and still flanking the target, so basically a semicircle around the target), that's how I got the achievement at least.