How can I install IOS3 to my old iPhone3G?
The erase operation is easy. 


settings app, general, reset, erase all content and settings


Breaking Apple's software update mechanism to put an older iOS on the device is going to cost time to gain expertise and even if you invested hours learning how jail breaking works, you might not be able to revert the OS. Running a jailbreak on more current OS and hardware is easier, but still something quite technical. Going that far back is less likely to succeed if reading the how-to isn't giving you a clear picture how to start to jailbreak your device in question. 

I would do the easy and get her a wiped device on the OS that iTunes will support rather than needing to keep jail breaking it if the user chooses to update things.