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

Code 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)(GRYB) Unlock Everything (no sound plays when you enter these chords)

Through the Fire and the Flames - To unlock TTFAF, you must beat Career mode on any difficulty. Then, when the credits role, you will play TTFAF (note: you can't fail). From then on, TTFAF can be found in the bonus songs section.

Through the Fire and the Flames Beat Career Mode on Any Difficulty

Unlockables

Lou Beat his Boss battle, Buy in Shop
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 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 Guitar 5* every song in career on hard mode