https://hard-resets.com/iphone-3gs-hard-reset/
From DFU Mode
Sometimes, hard resetting the iPhone 3GS doesn’t do the trick. What you need to do in this case is to enter DFU mode ( Device Firmware Upgrade ) that allows you to restore the device from any state. This is a bit tricky so please follow these steps thoroughly:
DFU Mode
How the iPhone 3GS looks in DFU Mode
Make sure the device is turned off and that it is not connected to the PC.
Hold down the Home button and connect the 3GS to the computer ( don’t release the Home button yet ).
Once the iTunes and usb cable logo appears on the iPhone appears, you can let go of the Home button.
Restore the device using the iTunes method from above.
There may be some reasons for your device to be bricked. The most important thing is to make sure you have enough battery when upgrading to a newer software version, because interrupting the upgrade will lead to a bricked iPhone 3GS and a reset to factory setting is needed.