Once you beat the game, you'll unlock the "Extra New Game" and "Extra Costume" options. The costumes are only for an Extra New Game. You have to enter a password to unlock a costume. Many costume passwords are displayed after you beat the game in the ranking screen or in the game in memos. The codes are case sensitive so make sure you enter them correctly. Below is every single code for the game -- most of them are found only on the internet or in gaming magazines:

"Silent Hill 3 Original" Costume Passwords (PC / PS2)
"Block head" Shirt - PutHere2FeelJoy
"Don't touch" Shirt - TOUCH_MY_HEART
"God Of Thunder" Shirt - GangsterGirl
"Golden Rooster" Shirt - cockadooodledoo
"Heather" Shirt - HappyBirthDay
"Killer Rabbit" Shirt - BlueRobbieWin
"Onsen" Shirt - I_Love_You
"Royal Flush" Shirt - 01_03_08_11_12
"The Light" Shirt - LightToFuture
Transform Costume - PrincessHeart
"Transience" Shirt - ShogyouMujou
"zipper" Shirt - Shut_your_mouth

The Rest (PS2 only)
"13eme RUE" Shirt - Suspense
"EGM" Shirt - EGMpretaporter
"Game Informer" Shirt - gameinformer
"GamePro" Shirt - ProTip
"GAMEREACTOR" Shirt - SH3_Wrestlarn
"" Shirt - iaml33t
"GMR" Shirt - GMRownzjoo
"GN" Shirt - IwannabeaGJ
"IGN.COM" Shirt - IGN_pickleboy
"OPM" Shirt - SH3_OPiuM
"OPS2" Shirt - extra_thumbs
"Play" Shirt - sLmLdGhSmKfBfH
"PSM" Shirt - badical

There are a total of 26 extra costumes, plus Heather's original costume. All costumes with "Shirt" in their name come with a shirt, blue jeans, wristbands (some have their own colours) and shoes. However, the "God of Thunder" Shirt is an exception -- it still comes with a shirt, wristbands and shoes, but it also comes with brownish jeans, and Heather is covered with tattoos and scars. She also has gray hair.

In an Extra New Game, you'll be wearing a new shirt if you entered a code in, and all the other shirts will be in your inventory under Items. You can wear any shirt that you have and you can change it whenever you want. To wear the original costume, unequip the current costume you're wearing.

Extra Costume Clues
In an Extra New Game, there will be a code in a magazine in the Women's Locker Room on 2F of Brookhaven Hospital that you'll probably find or already have found. There is another one though. After you complete an Extra New Game and play another on Hard Riddle Level, there will be an extra riddle in the Library of the Church. Try to solve it yourself. If you can't, the answer is above in the costume part of this section -- it's the code for the "Royal Flush" Shirt.