Below is a list of cheats for RCT3. Note that for all the cheats to work, you need the Latest Update.
Rabbit - Animals Constantly Breed - Should make your animals reproduce very quickly, like rabbits! (Wild! Only)
Alvin Swazonegger - Allow Large Billboard Movies
Jurassic Dart - No information, but is a valid cheat in the code.
M Brookes - Enables you can place wall pieces and other scenery items on the edge of path tiles.
Alistair Lindsay - alters the speed at which some sound effects are played at pause, fast and fastest.
Rick Griffiths - changes the inside of a tunnel from rubble to sharks on both sides, the ceiling and the floor.
Mornington Crescent - This allows building shops etc underground (use SHIFT to lower an object's position until it is say, level with an underground path).
David Walsh - This allows removal of the user interface (ie all the icons, menus etc) using CTRL-U once the cheat is active. This is great for taking screen shots. Press CTRL-U again to get the user interface back.
Isambard Kingdom Brunel - Trains Staff (Costs you about $1000 plus the total cost needed to fully train all of your Staff Members)
FPS - Displays an Frames Per Second counter.
Ghost Town - Will stop the flow of peeps coming into your park. This seems irreversible and the only way to disable it is to shut down the game fully.
Andrew Thomas - decreases track friction, which can make coasters go faster along their tracks
David Braben - Disable the Lift Chain Speed & Launch Speed limits. Normally, you are not permitted over a set speed limit. Usage: Select a coaster. Select "Operating Mode". You must have "Powered Launch" selected to alter the Launch Speed.
Andrew Gillett - Unknown effect (On the first Career mission, this code will double the value rating of your park - which will complete your first mission objective. On other tested missions, it seems to possibly cause a slight increase in park value too)
John D Rockefeller - Increase your money by $10,000
Chris Sawyer - Cheer (Lp) - All guests applaud you & Jump into the air
Mouse - They follow the mouse cursor. They start off looking at the ground because the cursor is at the bottom of the screen. Wave it around a bit and the peeps watch!
Guido Fawkes - Enables the "Advanced firework editor". Usage: Scenery -> Firework Mixmaster -> Fireworks Displays -> Add firework display -> Advanced firework editor.
Atari - Cheer (Fade) - All guests applaud you
James Hunt - You are given a buggy vehicle to ride in - exit coastercam and delete as scenery when done
D Lean - Open the "Flying Camera" routes editor - You can also use CTRL-SHIFT-0 (the number zero) For more info click here .
A Hitchcock - Hitchcock - not sure what this does (could be related to the birds like Alfred's movie)
PhotoStory - unknown code
ATITech - Every Person (both guests & staff) move super fast, though the game time passes at normal speed (rides & coasters do not speed up) (Expires after 20 seconds)
Make Me Sick - All guests become Sick and vomit immediately (Repeat it to make the park insanely dirty - watch them handymen run). Good for scenario creation
Jonny Watts - PeepCam (see the world through the eyes of the guest who you just renamed)
John Wardley - No Coaster Height Limit (ride & coaster tracks can be built without height limitations)
Frontier Unbreakable (Buildings/Rides/Coasters do not break down any longer)
Jon Roach - Ride All Rides (make all guests ride every parkride before leaving the park) OR, it may just make guests ignore their nausea rating
Sam Denney - Ride All Coasters (make all guests ride every park coaster before leaving the park) OR, it may just make guests ignore their nausea rating
Atomic - Causes "nuke" shockwaves on coaster crashes.
Shifty - Makes the peeps dance