Installing USB driver is a must to make communication and connection between your Galaxy S3 & PC or Mac. You will not able to connect your and talk to Galaxy S3 if you need to flash zip files or install custom ROM from PC or Mac. Samsung Galaxy S3 USB driver is not bundle in the android SDK developer pack from Google and you need to install it separately via Samsung Kies. Samsung Kies is a tool which bundles with USB driver, Bluetooth drivers and other drivers needed to communicate to Galaxy S3 and PC/Mac. Besides that, via Samsung Kies, you will be able to sync your data, photos & contacts to PC/Mac.
Tutorial on Samsung Kies will be posted soon but before that, make sure you download Samsung Kies in your PC/Mac.
Download Samsung Kies For International Version | 89.56MB
[Via Samsung Kies]