After you complete the barn segment in Life Is Strange, you get to a big vault door beneath the barn. This door requires a 3 digit code to open, and the solution to the puzzle is not always obvious to players.
There are besically two methods to solve this puzzle. If you just want the code, scroll down below these to save yourself the trouble of reading all about it. Anyway, the first way to get the code is to look at the notes from Nathan. These are on the same paper you looked at in the evidence section earlier when trying to figure out his PIN. Anyway, there's only a single 3 digit code among these, so use this to open the vault door. You can find the paper by going to the journal and look at all the documents you have found.
The other method is a bit more brute force based. Basically look at which numbers on the door that has been worn down. You will quickly see that the numbers 2, 4 and 5 have been used a lot more than the others. Now just try every combination of these until you manage to open the door, which should not take too long.
And if you just want the solution; the code is 5-4-2. Type that in and the vault door beneath the barn will open up.