Q: Old iPhone Forgotten Password

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

This may work on your 3GS.  If not, I'm not sure anything will.  It will erase all of the data on the phone.

If your iPhone can still connect to any computer with iTunes (via cable) you can restore it to it's factory defaults with recovery mode.

Link to page: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

(Let me know if this works with your old gen iPhone).

Erase your device with recovery mode

If you've never synced with iTunes or set up Find My iPhone in iCloud, you'll need to use recovery mode to restore your device. This will erase the device and its passcode.

1. Connect your iOS device to your computer and open iTunes. If you don't have a computer, borrow one from a friend, or go to an Apple Retail Store or Apple Authorized Service Provider.
2. While your device is connected, force restart it: Press and hold the Sleep/Wake and Home buttons at the same time. Don't let go when you see the Apple logo — keep holding until you see the recovery mode screen.
   
3. When you see the option to Restore or Update, choose Restore.
   
   iTunes will download software for your device. If it takes more than 15 minutes, your device will exit recovery mode and you'll need to repeat steps 2 and 3.
   
   
4. Wait for the process to finish. Then you can set up and use your device.