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!