Boxes with combination locks are found randomly in the game looting spots, houses, garages, barns. It is a short long box,light blue in color with 4 combination numbers. To find the right number keep scrolling each one of them until your controller vibrates and you hear a lock picking, do that for all 4 of them and boom, you unlocked your box, do it 3 times and BPPM, Achievement unlocked.
Note, this achievement has tracking progress to it, so you will know how many more you need, even tho you can probably count to 3 [