Replacing the glass on an S3 is actually quite easy. Ive dont it to my sisters S3 twice (she dropped it again just a week after the first fix).
precedure is quite simple use a hairdryer to melt the glue holding the top glass in place. once the glue is suitable melted just lift of the old glass (if the old one is shattered as appose to cracked you may need to spend some time scraping) then apply the new screen.
Plenty of youtube vids like the one posted above
