This achievements is split in 2 parts. For the contract assassination (these are the Window, Roof and Nun Assassinations) these can only be given to the player by the Suspicious Man in the back of the Tavern in each Port.
As for the "Triple Murder" mission, this is given to the player from a captain, and it will require the player to have a good proficiency with the bow, as this mission requires stealth to complete.
Once both parts are complete, the Achievement will shortly afterwards unlock.