Tony Hawk's Pro Skater 3

Unlockable: Everything
Pause the game and go to the Cheats selection in the Options menu. Once there, enter the code "backdoor" to unlock everything.

Unlockable: Level Select
Pause the game and go to the Cheats selection in the Options menu. Once there, enter the code "roadtrip " to enable level select.

Unlockable: (Almost) All Skaters
Pause the game and go to the Cheats selection in the Options menu. Once there, enter the code "YOHOMIES" to unlock every skater except Doom Guy.

Unlockable: All Created Skaters
To unlock all of the created skaters (the dudes made up by the developers), enter this code in the Cheat screen: WeEatDirt.

Unlockable: All Decks
To get all the decks in the game without doing a lick of work, enter this code at the Cheats screen: GiveMeSomeWood.

Unlockable: Cheat List
Unlock the corresponding cheat by beating the Career Mode 100% the indicated number of times:

1: Darth Maul (Skater)
2: Wolverine (Skater)
3: Officer Dick (Skater)
4: Warehouse Level (From THPS1)
5: Private Carerra (Skater)
6: Burnside Level (From THPS1)
7: Ollie the Magic Bum (Skater)
8: Kelly Slater (Skater)
9: Roswell Level (From THPS1)
10: Demoness (Skater)
11: Snowboard Mode (Cheat)
12: Unlimited Special (Cheat)
13: Perfect Rail Balance (Cheat)
14: Super Stats (Cheat)
15: Huge Skater (Cheat)
16: Slow Motion Mode (Cheat)
17: Perfect Manual Balance (Cheat)
18: Small Skater (Cheat)
19: Moon Physics (Cheat)
20: Expert Mode (Cheat)
21: Eyeball Man (Skater)
22: First Person Mode (Cheat)

Unlockable: Max Stats
Pause the game and go to the Cheats selection in the Options menu. Once there, enter the code "PUMPMEUP" to unlock max stats.

Unlockable: All Movies
Pause the game and go to the Cheats selection in the Options menu. Once there, enter the code "Peepshow" to unlock all movies.