here’s a trick that can be applied to all kinds of WiFi issues on Galaxy S3:-
1. Go to your GS3’s dialer and dial *#0011#
2. Now you have to hit the left menu button when you see the “service mode” screen
3. Choose WiFi
4. Turn off the “WiFi power save mode” button if its on
5. Now exit this menu. Turn off and turn on your WiFi. Re-enter the network info if you have already entered it before.
**hope this trick would give you some relief**