This guide will only be used for Carlos's part in the RPD. That's where you'll find the only 2 lockers with padlocks in Resident Evil 3.
One of the lockers is located on floor 3F of the police station, following the corridor up the stairs. The code is simply located in the locker room on 1F, at the far end, on a wall. There will be a picture there with the padlock that says "DCM". The locker contains assault rifle ammunition.
The second and last locker of the game is also in the RPD, but this time on floor 2F, in the locker rooms just before the showers.
You can find the code in the room just south of the Operations Room on 1F, written on the whiteboard: "CAP". This locker contains a flash grenade.