AT&T and Verizon aren't the only carriers getting in on the pre-order action. You can pre-order the Sprint version as well. Additionally, Sprint has something available that some of the others do not show yet. You can also preorder the 32GB version of Sprint's SGS3 for only $50 bucks more than the 16GB version. The 16GB version is $199.99 and the 32GB version is $249.99.
Be sure to hit the source link below to setup your preorder.
Source: Sprint - Preorder for 32GB Version of Samsung Galaxy S III or Sprint - Preorder for 16GB Version of the Samsung Galaxy S III