You need to troubleshoot the problems you are having with your iPhone if it is locking up, freezing, or not running applications (apps) properly and part of the troubleshooting can involve how to reset iPhone 3. The first step you should take if your iPhone is locked up or the screen has frozen is to restart the phone. After you have done that, you should force a shut-down of any apps that are causing a problem. If you still have not solved the issues, you should consider a reset. When you reset your phone you will not lose data nor cause damage to any of your files.
With your iPhone on, hold both the on/off button and the home button on the device at the same time. Continue holding and do not turn the slider off that appears on the screen. Continue holding both buttons for up to ten seconds until you see the logo for Apple appear on the iPhone screen. Now you can release those two buttons because the iPhone has been reset. If this has not solved the problem that you had with the phone you will need to restore the device using an iTunes program online.