Samsung Galaxy S3 WiFi Connectivity Problems with SOME Wireless Networks
I have my own Galaxy S3 and I found that this solution worked great for me:


  
  Go to your GS3’s dialer and dial *#0011#
  Now you have to hit the left menu button when you see the “service mode” screen
  Choose WiFi
  Turn off the “WiFi power save mode” button if its on
  Now exit this menu. Turn off and turn on your WiFi. Re-enter the network info if you have already entered it before.