The image to the right gives no clue as to how to get there so look directly on the map, down from above and one will be able to see some paths running up into the mountains, starting a short distance east of Falkreath, near Peak's Shade Tower.
To get to the crates follow the path until one reach the stacked rocks with a few bear traps next to them on the right, but don't turn here (leads to Angi's Camp). A little bit further on the main path will take a hard turn to the left, after this turn keep an eye on the rock wall on the right, after about 15-20 seconds of walking one will see an opening going up to the right through the rocks, follow this cliff-top traveling directly west.
After about another 15-20 seconds one will reach the top of the incline and the crates will right there where it levels out. A short walk south from the crates one can find a gold ore vein.
- Three Thieves is a rare book with only two other known locations in Skyrim, and one in Apocrypha, in the Dragonborn expansion.