This is to unlock the Carnage a Trois outfits for Braddock, Daisy and Yeti.
complete each characters personal operation.
Baddock - Lets go
Daisy - The Morning After
Yeti - Caged Ice
when all of the above have been completed you will see a new operation called Carnage a Trois. Complete this and you will unlock the outfit for each character.
Once you have the outfits ,equip them on each character in the loadout screen before you deploy to unlock the achievement.