Every Android phone has a safe mode just like we have in windows. This Safe mode can help you uninstall any app which is causing instability of the system. Once you boot your phone in Safe mode it will only run with Stock Apps comes with the phone however you have option to uninstall any downloaded app from App Manager.
To enter in to safe mode in Galaxy S3
Switch off your phone Completely
Switch it on using power button
once you see Samsung logo start pressing the the left touch button at bottom..
You will see you phone will boot in safe mode and a safe mode text will be watermarked on bottom left.
To come out of Safe mode in Galaxy S3 ... Just reboot your phone normally.
Hope this will help