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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

Hyperspeed
OBOYOBOY

Unlock all songs in Quick Pla
(YO) (RB) (RO) (GB) (RY) (YO) (RY) (RB) (GY) (GY) (YB) (YB) (YO) (YO) (YB) Y R (RY) R Y O

Performance Mode
(RY) (RB) (RO) (RB) (RY) (GB) (RY) (RB)

Precision Mode
(GR) (GR) (GR) (RY) (RY) (RB) (RB) (YB) (YO) (YO) (GR) (GR) (GR) (RY) (RY) (RB) (RB) (YB) (YO) (YO)

Air Guitar
(BY) (GY) (GY) (RB) (RB) (RY) (RY) (BY) (GY) (GY) (RB) (RB) (RY) (RY) (GY) (GY) (RY) (RY)

No Fail (does not work in career mode)
(GR) B (GR) (GY) B (GY) (RY) O (RY) (GY) Y (GY) (GR)

Large Gems
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)

Easy Expert (does not work in Career mode)
(GR) (GY) (YB) (RB) (BO) (YO) (RY) (RB)

Unlock Everything (no sound plays when you enter these chords)
(GRBO) (GRYB) (GRYO) (GYBO) (GRYB) (RYBO) (GRYB) (GYBO) (GRYB) (GRYO) (GRYO) (GRYB) (GRYO)

Unlockable Characters

Slash
Beat his boss Battle, Buy in Shop for $10,000

Tom Morello
Beat his Boss Battle, Buy in Shop for $10,000

Lou
Beat his Boss battle, Buy in Shop for $15,000

Additional Guitars

Moon Shot Guitar
Beat Career Mode on Easy

Bat Guitar
Five Star every song in Career Mode on Easy

Saint George Guitar
Beat Career Mode on Medium

Jolly Roger Guitar
Five Star every song in Career Mode on Medium

Rojimbo Guitar
Beat Career Mode on Hard

Nemesis 13 Guitar
Beat Co-Op Career Mode on any difficulty

El Jefe Guitar
5-star all songs in Expert mode.

Risk Assessment Guitar
Complete Expert mode.

Radioactive
Beat Co-op Career mode on Hard

Pendulaxe
Beat Co-Op Career on Expert

Beach Life Bass
Beat Co-Op on Hard

Tiki Guitar
5* every song in career on hard mode

Neversoft Skateboard Guitar
5 star every song on Expert Co-Op Career

Tiki Bass
Beat Co-Op on Easy

(Cheat Via: GameSpot)