how i download iBooks in 3GS

Hi,

iBooks is a built-in app on iOS 8 or later. If you have an earlier version of iOS, you can download the iBooks app for free from the App Store on your iPhone, iPad, or iPod touch. On Mac, iBooks is included with OS X Mavericks or later.

About iBooks - Apple Support

An iPhone 3 cannot update to iOS 8.