As you advance Resident Evil 3 you eventually find yourself playing as Carlo as he makes his way through the Raccoon City Police Department. When you explore enough, you may find yourself trying to unlock a locker protected by a dial and you need a three letter code to unlock it.
The location of the locked locker is on the third floor. You can access it by going to the STARS office on the second floor. Before going to the office, go up the stairwell. When you reach the top you will find it at the end of the hallway on your right.
To find the solution for the locker code, you need to enter the key that you find above the boxes just before going to that locker. When you have this key, go back to the first floor and use it to unlock the door to that room. Kill the zombie in the room, then look at the photo attached to the whiteboard. The code is written on it, which should read: DCM. Go back to the third floor and enter the code to access 20 Assault Rifle Shells.
There are a handful of zombies and other gear you can catch along the way as you run around the police station. Make sure to grab them and watch out for the lick as you make your way to the first floor to unlock the security deposit room.