As its name suggests, the download mode allows you to download and flash ROMs and kernels on your device. Entering the Galaxy S3 i9300 download mode is easy, though you should know that it's dangerous doing anything there and we first have to let you know the risks and what you should be careful about.
The download mode can very easily brick your device if you are not careful. ROMs and kernels are usually flashed with Odin, which, if configured poorly, can permanently brick your Galaxy S3. You have to always follow the guide and all the steps before beginning the flashing process.
Flashing anything in download mode can also void your Galaxy S3 warranty, so you have to be aware of this before doing it.
Please note we cannot be held responsible for any damages caused to your device by entering download mode and flashing anything.
How to enter download mode on Galaxy S3
Like I said before, it's very easy and you only have to use a three-keys combination.
First shut down your device, then power on by simultaneously pressing the power button, home button and volume down key. Now you will see download mode and you will be asked if you want to cancel or continue. Press volume up to continue or volume down to cancel and reboot.
In order to flash anything on your device, you will need to download Odin.
If you want to enter the recovery mode on your Samsung Galaxy S3 i9300, then here's a guide.
We also have a video tutorial for entering download mode: