In case anyone is having trouble with understanding what and where to find these challenges I will explain:
When you start a Templar hunt you will see one or more optional objectives (I don't know if these are random or always the same). The objective(s) will usually be something like "hide the body in the water" or "kill target with the hidden blade" and the objective will usually appear in the top corner. Just make sure to not fail that challenge and succeed in killing the target and escaping the area. Once you do this 3 times the cheevo will pop.
Please note: Unless someone can confirm otherwise I don't believe its possible to redo Templar hunts meaning that if you already did them all and didn't get the cheevo you may have to start a new game.
Thanks to some great feedback from Basileus and Master AO, Ive decided to just highlight important information about the Templar Hunts.
1) You can redo Templar Hunts: You can do this through the progress tracker. If you did any Templar Hunts but failed the challenge you can retry it.
2) Redoing Templar Hunts DOES NOT VOID the achievement.
3) Challenges are always the same for each specific Templar Hunt: For example if you are doing a Templar Hunt and the challenge is "Kill target with a hidden blade" than it will always have the same challenge even if you restart or redo it. If you are struggling to complete a particular challenge try another Templar Hunt.