There doesn't appear to be any cheat codes for the game that I can find. However, if you defeat Nemesis on Hard Mode he will drop something that gives you benefit.
- First Kill - Eagle Parts A
- Second Kill - Eagle Parts B
- Third Kill - First Aid Spray Box
- Forth Kill - M37 Parts A
- Fifth Kill - M37 Parts B
- Sixth Kill - First Aid Spray Box
- Seventh Kill - Infinite Ammo for One Weapon
It appears as though you must beat the game several times, and on different difficulties, to unlock everything.
In reference to the link that you have added: What you are seeing on that website would be Hex Editing saved game files to give you items or change things around. It is much easier to hex edit saved game files with the PC version of the game than the PS version. The two versions share the same gamesave file format, but it would require much more effort to move a game save between your Playstation and a PC.
If you are playing on a PC here is a link that you might find helpful:
RE3 Hex Editing - GameFaqs Guide
If you are playing on a PSX, PS2, or PS3 then this will quickly become a how to guide that you can probably find elsewhere.