With a simple hack, you could enable Internet Tethering and MMS Support in iPhone OS 3.0 firmware. But Apple has blocked this hack from working in Firmware 3.1.2.
But no need to get disappointed because you could enable Tethering in your iPhone if you have a jailbroken iPhone with you. If you haven’t done that, then simply follow the jailbreaking guide posted here – with PwnageTool 3.1.4 and with Blackra1n in Windows and in Mac.
After jailbreaking your iPhone, just follow the steps given below to enable internet tethering in your iPhone OS 3.1.2 Firmware.
1. Go to Cydia, search for MyWi app and install it. MyWi will install RockApp along with it. If it doesn’t, then do it manually.
2. Run the RockApp and Â get a Rock ID.
3. Using this ID sign in in RockApp and you could notice an update for MyWi.
4. Run it and reboot your iPhone when finished.
5. Launch MyWi and enable USB/Bluetooth Tethering.
6. Now go to Settings -> General -> Network -> Internet Tethering and turn on Internet Tethering.
That’s all. Now you have enabled internet tethering in your iPhone OS 3.1.2 firmware.