The Texas Chain Saw Massacre

You’ll need to find a bone pile to get the bone scrap/knife and then get behind a family member and hit RT to use it. Repeat 2 more times and welcome your achievement! Keep in mind if you attack one from the front it starts a “fight” instead of a sneak attack

Get a bone knife by searching bone piles (mostly found on basement), then get behind a Family Member and use the knife by pressing RT, repeat three times and the achievement is yours, there are some ways to make this easier:

1. Use Leland and his Life Saver ability, simply knock down a family member and get behind them, stay on their back and spam RT to stab them.

2. Get 2 bone knifes and look for any family member except Leatherface, use RT to stab them front face and initiate a "close encounter", you'll have to rapidly press A to win it and stun them, once they're stunned simply get on their back and spam your RT, this way you will stab them immediately after they recover from your stun.

3. Simply look for each family member and stay close to them, once they initiate any animation run towards their back and use your knife.

These tips also work for the "Now it's my turn achievement"