Hard Reset SAMSUNG i5800 Galaxy 3

First method:

1. First, turn off the phone by using the Power button.
2. Now press and hold together: Volume Up + Volume Down + Home + Power button for about 10 seconds.
3. Release held keys when the Samsung Logo appears on the screen.
4. Then select from Recovery Mode menu "wipe data / factory reset" using Volume buttons to navigate and Power button to confirm.
5. Choose "Yes -- delete all user data" to confirm the whole operation.
6. After that select option "reboot system now".
7. Well done! The hard reset has just been performed.

Second method:

1. Start by turninng on the device.
2. From the Home Screen go to Menu and then to Settings.
3. Tap Privacy and then Factory Data Reset.
4. Afterwards select Reset Phone
5. To confirm your choice select Erase everything.
6. That's all.

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.