As you make your way through 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 attempting to unlock a locker protected by a dial lock, and you need a three-letter code to open it.
The location of the locked locker is on the third floor. You can reach it on your way to the S.T.A.R.S office on the second floor. Before proceeding to the office, continue 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 grab the key you find on top of the boxes just before you walk up to this locker. When you have that key, return down to the first floor and use it to unlock the door to that room. Kill the zombie in the room, and then look at the photo attached to the whiteboard. The code is written on it, which should read: DCM. Return to the third floor and input the code to gain access to 20 assault rifle rounds.
There are a handful of zombies and other gear you can grab on the way while you run around the police station. Make sure to grab them, and watch out for the licker when you make your way to the first floor to unlock the Safety Deposit room.