Problem: I just updated to Android 4.3 with my Samsung Galaxy S3 and just today, immediately after the update, I was getting the Play Store error 403 when I tried downloading a free application. I don't know what the problem really is but it just started. I am running a fresh OS here and I haven't downloaded any app either. Why is this problem happening?
Answer: You are not the only one experiencing the Google Play Store Error 403 and it is not even Galaxy S3-specific problem. The fact is, it started a week ago. It seems there are some problems with Google's servers and that's where the problem starts. If you do a quick search online, you will be able to find hundreds, if not thousands, of users complaining about this specific issue. Unfortunately, there were no updates or official statements issued by Google ever since the problem started.
Many said their problem were fixed without doing anything, so that clearly shows it's not with your phone. My advice is, wait it out to see if your problem will be fixed in the next 24 hours. If not, then try to clear the cache and data of the Google Play Store on your Samsung Galaxy S3. If that doesn't work either, clear the cache and data of Google Framework Services. Furthermore, uninstalling Play Store updates may also help resolve the issue.