Can I use Wi-Fi Personal Hotspot with the iPhone 3GS?
Your iPhone 3GS will not share its Internet connection via Wi-Fi. Only the iPhone 4 supports this feature. That being said, the Personal Hotspot setting on the iPhone 3GS is really just the old Internet tethering app with a new name. You can share your cellular data connection on the iPhone 3GS with other devices using Bluetooth or a USB cable.
Sharing your iPhone 3GS cellular data connection with other devices via Bluetooth requires iOS 4.3 or later. Before using the Personal Hotspot you must activate a DataPro plan with tethering from AT&T Mobility on your wireless account.