Call of Juarez: Gunslinger

You can unlock this achievement right away from the first mission !

In the first mission there are 3 Nuggets of Truth:
1. Lincoln County War
2. Stinking Springs
3. Pat Garrett

Check the following video for the exact location of each one.

Easiest way to do this is obviously the very first mission.

First nugget is a piece of paper, it's basically unmissable and it's located near the start of the mission.

Second nugget is in a separate room when you go to Billy the Kid about halfway in the mission.

The third one can be a little tricky, because it does not always detect your view well, but it's located right before the final barn, where you go for the horses. It's a white hat on a right sided scarecrow in front of the barn.

Hope this helps!