Just like in the previous game, in Resident Evil 3 you can find safes and lockers with combination locks on the levels. The solution is usually somewhere nearby, but searching for it is not necessary. Knowing the right code, you can open the lock without wasting your time searching.
Notably, almost all safes and locks are found in episodes with Carlos. Jill will only have one safe in the warehouse. On the other hand, Jill can open yellow locks that Carlos cannot.
The first safe you will find is in the warehouse, located on the second floor of the building next to the café. There will be a note with a hint near the safe. You can find the solution in the nearby pharmacy. Carefully examine the poster at the very end of the pharmacy.
When the episode with Carlos begins, the hero will find himself in the familiar police station. On the first floor, you can find a safe in the west office of the police station. The code is written in a note located in the safe room.
On the second floor in the shower room, you can find a locker with a combination lock. The code is written on the board in the briefing room.
The last locker with a combination lock is located on the third floor of the police station, at the very end of the corridor. The code is indicated on a photograph attached to the board in the evidence storage room.
The safe is located at the nurse's station. It's not in a very visible place, so it can be easily missed.