Real cheats for GH3:
To enter these: On the main menu,go to the 'Options' part,go down to 'Cheats',choose 'Enter Cheat',put fingers in the note combos and strum. They should sound like bits from random songs.
Large Gems: (makes notes extremely huge;I really don't see the point of this one)
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
Hyperspeed: (makes everything go superfast)
O, B, O, Y, O, B, O, Y
Preformance Mode: (makes fretboard,nots,meter,and score disappear. You'd have to memorize an entire song to play it,and you can still fail. Not much point and very hard)
RY, RB, RO, RB, RY, GB, RY RB
Precision Mode (youhave to hit notes exactly when the come,no sooner,no later. Very difficult)
GR, GR, GR, RY, RY, RB, RB, YB, YO, YO, GR, GR, GR, RY, RY, RB, RB, YB, YO, YO
Unlock All: (unlock all the songs)
YO, RB, RO, GB, RY, YO, RY, RB, GY, GY, YB, YB, YO, YO, YB, Y, R, RY, R, Y, O
Air Guitar (makes the guitar disappear,truly,an air guitar. Mostly for a cool effect.)
BY, GY, GY, RB, RB, RY, RY, BY, GY, GY, RB, RB, RY, RY, GY, GY, RY, RY
Easy Expert: (*supposedly* makes expert level easier. It's still not easy for me.)
GR, GY, YB, RB, BO, YO, RY, RB
Never Fail (self explanitory. Only works in Quickplay!!!! It doesn't work in career mode. Probably my favorite cheat)
GR, B, GR, GY, B, GY, RY, O, RY, GY, Y, GY, GR
Unlock Everything (unlocks,songs,characters,guitars,outfits,anything you could possibly buy. I love this one)
GRBO, GRYB, GRYO, GBYO, GRYB, RYBO, GRYB, GYBO, GRYB, GRYO, GRYO, GRYB, GRYO
ONLY FOR XBOX360:
Bret Michaels: (has character turn into Bret Michaels. Never tried it because I have a Wii,my cousin gave me this one)
GR, GR, GR, GB, GB, GB, RB, R, R, R, RB, R, R, R, RB, R, R, R

Phew! It took some research for that! Hope it might help with something!