Breaking a Locker

It seems like lockers aren't as secure as we think. There seems to be 60 x 60 x 60 = 216000 combinations, but in reality, the margin of errors make it only 20 x 20 x 20 = 6000 combinations.
Every number has a margin of error. It could be 2 back, 1 forward, 1 back, 2 forward, or 3 back. Smart people would just try multiples of 3. Another thing also make it easier, and that is, on the third number, you can keep turning and trying, until you get to the correct combination (of course if you turn 2 times clockwise, it will reset).
For example, if a locker combination is 26, 7, 38, you can go for 24, 9, 35, and it will still open. Because of the property of the third number (you don't have to reset to try a different third number), we only have to focus on the first and second number. There are 20 x 20 = 400 combinations. It will take 1 minute to try the third number.
Therefire, a maximum of 400 minutes is required to crack a locker. Averagely, it will take 200 minutes, which is about 3 hours. If you are lucky, it might take less than 20 minutes.