Go to "options" and then go to "cheats." Now, cheats are entered completely differently in the third installment. You now have to strum with the frets on just like normal playing! Two or more notes in parentheses indicate a chord.
G=Green, R=Red, Y=Yellow, B=Blue, O=Orange
Cheat Effect

BY) (GY) (GY) (RB) (RB) (RY) (RY) (BY) (GY) (GY) (RB) (RB) (RY) (RY) (GY) (GY) (RY) (RY)	Air Guitar
(GR) (GY) (YB) (RB) (BO) (YO) (RY) (RB)	Easy Expert (does not work in Career mode)
OBOYOBOY	Hyperspeed
G R G Y G B G O G B G Y G R G (GR) (RY) (GR) (YB) (GR) (BO) (GR) (YB) (GR) (RY) (GR) (GY)	Large Gems
(GR) B (GR) (GY) B (GY) (RY) O (RY) (GY) Y (GY) (GR)	No Fail (does not work in career mode)
(RY) (RB) (RO) (RB) (RY) (GB) (RY) (RB)	Performance Mode
(GR) (GR) (GR) (RY) (RY) (RB) (RB) (YB) (YO) (YO) (GR) (GR) (GR) (RY) (RY) (RB) (RB) (YB) (YO) (YO)	Precision Mode
(YO) (RB) (RO) (GB) (RY) (YO) (RY) (RB) (GY) (GY) (YB) (YB) (YO) (YO) (YB) Y R (RY) R Y O	Unlock all songs in Quick Play
(GRBO) (GRYB) (GRYO) (GYBO) (GRYB) (RYBO) (GRYB) (GYBO) (GRYB) (GRYO) (GRYO) (GRYB) (GRYO)	Unlock Everything (no sound plays when you enter these chords)

Unlock Through the Fire and Flames

Beat Career mode on any difficulty

Unlockable Characters

Lou	Beat his Boss battle, Buy in Shop for $15,000
Elroy Budvis	Buy in shop for $10,000
Metalhead	Buy in shop for $10,000
Slash	Beat his boss Battle, Buy in Shop for $10,000
Tom Morello	Beat his Boss Battle, Buy in Shop for $10,000

Unlocking Additional Guitars

There are many guitars not in the store but are available for purchase after completing tasks.

Bat Guitar	Five Star every song in Career Mode on Easy
Beach Life Bass	Beat Co-Op on Hard
El Jefe Guitar	5-star all songs in Expert mode.
Jolly Roger Guitar	Five Star every song in Career Mode on Medium
Moon Shot Guitar	Beat Career Mode on Easy
Nemesis 13 Guitar	Beat Co-Op Career Mode on any difficulty
Neversoft Eyeball Guitar	5 Star Every Song in Co-Op Mode on Expert
Neversoft Skateboard Guitar	5 star every song on Expert Co-Op Career
Pendulaxe	Beat Co-Op Career on Expert
Radioactive	Beat Co-op Career mode on Hard
Risk Assessment Guitar	Complete Expert mode.
Rojimbo Guitar	Beat Career Mode on Hard
Saint George Guitar	Beat Career Mode on Medium
Tiki Bass	Beat Co-Op on Easy
Tiki Guitar	5* every song in career on hard mode