Here is my 100% Achievement Guide!
This is story related and cannot be missed as you need them to progress.
After knocking the owl from the perch, he drops the first coin.
Go left a few times until you see a pair of scissors sticking out of a tree.. Second coin is here (This is where i actually unlocked the achievement).
The third one you see a diagram on a tv, located next to a tree (This particular scene has 3 trees in it) so go left a few times until you see those same trees.. press A next to the middle and he digs it up.
Not sure if the achievement always unlocks after collecting the first two, but all 3 locations are there anyway.
Go to 22:30 to see the start on where they all are.
Hope this helps!