It depends on your carrier and your phone. Some carriers block tethering. Some do not. I know in the US, I have to pay for tethering on my iPhone. I could jailbreak and use MyFi from Cydia, but then my company would block me from corporate email.
You can try the app FoxFi on your S3 as it does not require root last time I checked. If it creates an ad-hoc access point, you may not be able to use WiFi as ICS cannot see ad-hoc for some strange reason. You can still use bluetooth, just pair the two, then on the A500 go into the settings for the S3 pairing (wrench or sliders to the right) and tick the "Use for Internet" box and it should be off and running.