iphone 3g unloced in itunes

Your initial question wasn't clear.

If you meant you wanted to backup or restore your iPhone with the screen locked with a passcode, you'll need to put your device into recovery mode. First, turn off your device and leave it off. Plug in your device's USB cable to a computer with iTunes. Hold down the Home button on your device as you connect the USB cable. Keep holding down the Home button until you see the Connect to iTunes screen. When you see that screen, release the Home button. If you don't see this screen, try steps 1 through 3 again. When your device is connected, iTunes will open. You'll see a message saying that iTunes has detected an iPhone, iPad, or iPod touch in recovery mode. Use iTunes to restore your device. Restoring in recovery mode will erase your device. If you previously synced with iTunes or iCloud, you might be able to restore from your backup.

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

If you mean you want to unlock the phone to use with a different carrier, follow these steps:

How to unlock your iPhone for use with another carrier - Apple Support