Songs (Normal)
To unlock songs in the normal fashion, simply pass the song successfully. It will thereafter be unlocked.
Through the Fire and Flames
To unlock the track "Through the Fire and Flames", beat the game.
Air Guitar
To unlock the Air Guitar, insert the following notes in the Options menu: Blue & Yellow, Green & Yellow x2, Red & Blue x2, Red & Yellow x2, Blue & Yellow, Green & Yellow x2, Red & Blue x2, Red & Yellow x2, Green & Yellow x2, Red & Yellow x2.
Guitarists
You can unlock Tom Morello (of Rage Against the Machine), Slash (of Guns N Roses/Velvet Revolver), Lou (the Devil in the game), and a character called MetalHead by beating them in the game and then purchasing them for $10,000 (Lou costs $15,000).
Boss Characters
To unlock any boss characters as playable character, simply beat them in the game and they will become unlocked.
Saint George Guitar
To unlock the Saint George guitar, complete Career Mode on Medium.
Performance Mode
To unlock Performance Mode, insert the following notes in the Options menu: Red & Yellow, Red & Blue, Red & Orange, Red & Blue, Red & Yellow, Green & Blue, Red & Yellow, Red & Blue.
Moon Shot Guitar
To unlock the Moon Shot guitar, complete Career Mode on Easy.
Precision Mode
To unlock Precision Mode, insert the following notes in the Options menu: Green & Red x3, Red & Yellow x2, Red & Blue x2, Yellow & Blue, Yellow & Orange, Yellow & Orange, Green & Red x3, Red & Yellow x2, Red & Blue x2, Yellow & Blue, Yellow & Orange x2.
Rojimbo Guitar
To unlock the Rojimbo Guitar, complete Career Mode on Hard difficulty.
Jolly Roger Guitar
To unlock the Jolly Roger Guitar, get five stars on each song during Career Mode on Medium difficulty.
Hyperspeed
To unlock Hyperspeed, go to the in-game cheat menu (found under Options) and insert the following cheat on your guitar: Orange, Blue, Orange, Yellow, Orange, Blue, Orange, Yellow.
Distant Visitor Guitar
To unlock the Distant Visitor Guitar, complete Career Mode on Expert difficulty.
Bat Guitar
To unlock the Bat Guitar, get five stars on each song during Career Mode on Easy difficulty.
All Songs (Quick Play Mode)
To unlock all of the game's songs for Quick Play Mode, insert the following notes in the Options menu: Yellow & Orange, Red & Blue, Red & Orange, Green & Blue, Red & Yellow, Yellow & Orange, Red & Yellow, Red & Blue, Green & Yellow x2, Yellow & Blue x2, Yellow & Orange x2, Yellow & Blue, Yellow, Red, Red & Yellow, Red, Yellow, Orange.
Nemesis 13 Guitar
To unlock the Nemesis 13 Guitar, complete Co-Op Career Mode on any difficulty.
Songs
The following songs can be unlocked in the game:
An Endless Sporadic - "Impulse"
Backyard Babies - "Minus Celsius"
Bret Michaels Band - "Go That Far"
Die Toten Hosen - "Hier Kommit Alex"
Dragonforce - "Through the Fire and the Flames"
Fall of Troy - "FCP Remix"
Gallows - "In the Belly of a Shark"
The Hellacopters - "I'm in the Band"
Heroes del Silencio - "Avalancha"
In Flames - "Take This Life"
Kaiser Chiefs - "Ruby"
Killswitch Engage - "My Curse"
LA Slum Lords - "Down N Dirty"
Lacuna Coil - "Closer"
Lions - "Metal Heavy Lady"
NAAST - "Mauvis Garcon"
Prototype - "The Way It Ends"
Revolverhead - "Generation Rock"
Rise Against - "Prayer of the Refugee"
Scouts of St. Sebastian - "In Love"
Senses Fail - "Can't Be Saved"
The Sleeping - "Don't Hold Back"
The Stone Roses - "She Bangs the Drums"
Superbus - "Radio Song"
PendulAxe Bass
To unlock the PendulAxe Bass, complete Co-op Mode on expert.
Beach Life Bass
To unlock the Beach Life Bass, complete Co-op Mode on hard.
El Jefe Guitar
To unlock the El Jefe Guitar, complete Career Mode on expert with five stars.
Neversoft Skateboard
To unlock the Neversoft Skateboard, complete Co-Op Mode on expert with five stars.
Tiki Guitar
To unlock the Tiki Guitar, complete Career Mode on hard with five stars.
Battle Lou in Easy Mode
To battle with Lou using Easy Mode notes, try the following. When the battle with Lou begins (on Expert Mode), strum without holding any frets down. When you lose, select the Tutorial option at the Game Over menu. Do the Guitar Battle Tutorial and play through it in its entirety. After the "Lesson Complete" message appears on-screen and the next fret board appears, pause the game and exit. This will bring you back to the Lessons menu. From here, press the red button, which will bring you back to the list of songs. The battle with Lou should be highlighted. Choose the song, and Lou's note chart will be set to Easy Mode now. Your note chart will still be in Expert Mode, but Lou's will be in easy mode.
Song Failure Secret Achievement
To unlock a special secret achievement, choose any song during Career Mode and then fail it ten times.
Unlock All Songs
The letters correspond to their color. Y = Yellow, R = Red, B = Blue and so forth.
From the Main Menu, go to Options, and into the cheat menu.
From there enter:
YO, RB, RO, GB, RY, YO, RY, RB, GY, GY, YB, YB, YO, YO, YB, Y, R, RY, R, Y, O
You should get a confirmation that this code worked.
Note: The songs will go away when you turn your Xbox 360 off, so you'll have to enter it again when you turn it back on.
Zero Point Achievement
Guitar Hero 3 has an achievement that can get you - points. Its absolutely worthless, but you can still say you've gotten one more achievement. To get it, go in career mode and go to any guitar battle you want. It's doesnt matter which one, but after you enter it, DO NOT accept. Instead, press the red fret button to decline. An achievement message will pop up because you have declined a boss battle.