http://apple.stackexchange.com/questions/6173/can-i-play-audio-from-my-iphone-on-my-mac
Simple, and simply surprising:
- Plug in your iPhone to your Mac using your Lightning Cable (doesn't work with 30 pin iPhones, sorry. [Yes I know you said 3GS])
- Launch QuickTime Player
- Choose File> New Audio Recording
- Click the hard-to-see down arrow next to the Record 'button', choose iPhone under 'Microphone'
- Play your music, game whatever on your iPhone, sound will come thru your Mac.
No need to hit record, the sound will play even if you don't record.