Install the latest Samsung USB Drivers on your Windows PC.
Before flashing, you need to collect a few free utilities and prepare your computer setup: Install the latest Samsung USB Drivers on your Windows PC
This is a soft bootloop. Pull the battery out to turn the phone off. Hold Volume Up + Home + Power to enter Recovery Mode. Use the volume keys to select Wipe Data / Factory Reset , confirm it, and then select Reboot System Now . Hold Volume Up + Home + Power to enter Recovery Mode
The journey to Android 4.3 for the Galaxy S3 was a story of anticipation and troubleshooting. After bypassing Android 4.2 altogether, Samsung finally began rolling out the Android 4.3 Jelly Bean update in November 2013. The first official build, I9300XXUGMJ9, was initially released for Vodafone customers in Ireland. This update was highly anticipated as it brought the Galaxy S III up to speed with many features from its successor, the Galaxy S4. After bypassing Android 4
Samsung Galaxy S3 GT-I9300 Android 4.3 Official Firmware: Best Download Link & Installation Guide
(Click "SamFW" for free slow speed, OR use Frija Tool on Windows)