Are you sure it was the SIM card you encrypted? I'd assume it was your microSD card that you encrypted.
If it was your microSD card, once you encrypted it the encryption key is stored in your S3, and it's tied just to your S3 so that's the only device that has full access to anything on that card. To return it back to it's original state, you need to copy all the data off the card using your S3, then just reformat it (and do not encrypt it). Copy all the data back onto it and it's back to it previous state so you can use it to transfer files back and forth again.
But if you did do something to the SIM card, (which typically you don't have any direct access to unless you've rooted your S3 anyway), you need to get a new one from your carrier.