Earning Money
To earn the money needed to buy the additional 25 songs, play through the "Career" mode of Guitar Hero 3. Completing a song will result in a cash payout. You'll earn more money for four- and five-star performances, and you'll also get more cash when playing at the higher difficulty levels.
Rock Shop
When you have earned enough money, go to the "Rock Shop" option in "Career" mode. Select "Songs" and scroll through the 25 bonus songs until you find one that you want. Choose the "Buy" option. You do not have to purchase all the songs at once; you can enter the "Rock Shop" at any point during "Career" mode.
Considerations
Any songs that you purchase are unlocked in the game's "Quick Play" mode. To play them, choose the "Quick Play" option and then press the yellow fret button when the main game songs are displayed. The screen will change to show your new songs.